diff --git a/.travis.yml b/.travis.yml
index 063bd58..2d89c8d 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,6 +1,6 @@
 language: ruby
 rvm:
-- 2.3.3
 - 2.5.5
+- 3.0.1
 before_install:
 - export TZ=Europe/Berlin
diff --git a/README.md b/README.md
index 9a1681e..ec0afcf 100644
--- a/README.md
+++ b/README.md
@@ -24,29 +24,38 @@ home's energy usage. That's how it started.
 
 ## Features
 
-* Build PDF reports based on [grafana](https://github.com/grafana/grafana) dashboards
-(other formats supported)
-* Include dynamic content from grafana (see [function documentation](FUNCTION_CALLS.md)
-as a detailed reference):
+* Build reports based on [grafana](https://github.com/grafana/grafana) dashboards, PDF
+(default) and many other formats supported
+* Easy-to-use configuration wizard, including fully automated functionality to create a
+demo report
+* Include dynamic content from grafana (find here a reference for all
+[asciidcotor reporter calls](FUNCTION_CALLS.md)):
   * panels as images
   * tables based on grafana panel queries or custom database queries (no images!)
   * single values to be integrated in text, based on grafana panel queries or custom
 database queries
 * Multi purpose use of the reporter
-  * webservice to be called directly from grafana - it also runs without further
-dependencies in the standard asciidoctor docker container!
-  * standalone command line tool, e.g. to be automated with cron or bash scrips
-* Comes with a complete configuration wizard, including functionality to build a
-demo report on top of the configured grafana host
-* Supports all SQL based datasources, as well as graphite and prometheus
+  * webservice to be called directly from grafana
+  * standalone command line tool, e.g. to be automated with `cron` or `bash` scrips
+  * seemlessly runs from asciidocotor docker container without further dependencies
+* Webhook callbacks before, on cancel and on finishing callbacks (see configuration file)
 * Solid as a rock, also in case of template errors and whatever else may happen
 * Full [API documentation](https://rubydoc.info/gems/ruby-grafana-reporter) available
 
+Functionalities are provided as shown here:
+
+Database | Image rendering | Panel-based rendering | Query-based rendering
+------------------------- | :-------: | :-----------: | :------------:
+all SQL based datasources | supported | supported     | supported
+Graphite                  | supported | supported     | supported
+Prometheus                | supported | supported     | supported
+other datasources         | supported | not-supported | not-supported
+
 ## Quick Start
 
 You don't have a grafana setup runnning already? No worries, just configure
 `https://play.grafana.org` in the configuration wizard and see the magic
-happen for that!
+happen!
 
 If your grafana setup requires a login, you'll have to setup an api key for
 the reporter. Please follow the steps
@@ -58,10 +67,6 @@ first.
 * [Download latest Windows executable](https://github.com/divinity666/ruby-grafana-reporter/releases/latest)
 * `ruby-grafana-reporter -w`
 
-Known issues:
-* images are currently not included in PDF conversions due to missing support in Prawn gem for windows;
-other target formats do work properly with images
-
 **Raspberry Pi:**
 
 * `sudo apt-get install ruby`
@@ -175,6 +180,6 @@ Inspired by [Izak Marai's grafana reporter](https://github.com/IzakMarais/report
 ## Donations
 
 If this project saves you as much time as I hope it does, and if you'd like to
-support my work, feel free donate, even a cup of coffee is appreciated :)
+support my work, feel free donate. :)
 
 [![paypal](https://www.paypalobjects.com/en_US/i/btn/btn_donateCC_LG.gif)](https://www.paypal.com/donate?hosted_button_id=35LH6JNLPHPHQ)
diff --git a/lib/VERSION.rb b/lib/VERSION.rb
index d906eeb..bb7a178 100644
--- a/lib/VERSION.rb
+++ b/lib/VERSION.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
 # Version information
-GRAFANA_REPORTER_VERSION = [0, 4, 0].freeze
+GRAFANA_REPORTER_VERSION = [0, 4, 1].freeze
 # Release date
-GRAFANA_REPORTER_RELEASE_DATE = '2021-04-14'
+GRAFANA_REPORTER_RELEASE_DATE = '2021-05-17'
diff --git a/lib/grafana/abstract_datasource.rb b/lib/grafana/abstract_datasource.rb
index 27b9a1e..30dafaa 100644
--- a/lib/grafana/abstract_datasource.rb
+++ b/lib/grafana/abstract_datasource.rb
@@ -6,33 +6,53 @@ module Grafana
   class AbstractDatasource
     attr_reader :model
 
+    @@subclasses = []
+
+    # Registers the subclass as datasource, which is asked by {#accepts?}, if it can handle a datasource
+    # model.
+    # @param subclass [Class] class inheriting from this abstract class
+    def self.inherited(subclass)
+      @@subclasses << subclass
+    end
+
+    # Overwrite this method, to specify if the current datasource implementation handles the given model.
+    # This method is called by {#build_instance} to determine, if the current datasource implementation
+    # can handle the given grafana model. By default this method returns false.
+    # @param model [Hash] grafana specification of the datasource to check
+    # @return [Boolean] True if fits, false otherwise
+    def self.handles?(_model)
+      false
+    end
+
     # Factory method to build a datasource from a given datasource Hash description.
     # @param ds_model [Hash] grafana specification of a single datasource
     # @return [AbstractDatasource] instance of a fitting datasource implementation
     def self.build_instance(ds_model)
       raise InvalidDatasourceQueryProvidedError, ds_model unless ds_model.is_a?(Hash)
-      raise InvalidDatasourceQueryProvidedError, ds_model unless ds_model['meta']
-      raise InvalidDatasourceQueryProvidedError, ds_model unless ds_model['meta']['id']
-      raise InvalidDatasourceQueryProvidedError, ds_model unless ds_model['meta']['category']
-
-      return SqlDatasource.new(ds_model) if ds_model['meta']['category'] == 'sql'
 
-      case ds_model['meta']['id']
-      when 'graphite'
-        return GraphiteDatasource.new(ds_model)
-
-      when 'prometheus'
-        return PrometheusDatasource.new(ds_model)
+      raise InvalidDatasourceQueryProvidedError, ds_model unless ds_model['meta'].is_a?(Hash)
 
+      @@subclasses.each do |datasource_class|
+        return datasource_class.new(ds_model) if datasource_class.handles?(ds_model)
       end
 
-      raise DatasourceTypeNotSupportedError.new(ds_model['name'], ds_model['meta']['id'])
+      UnsupportedDatasource.new(ds_model)
     end
 
     def initialize(model)
       @model = model
     end
 
+    # @return [String] category of the datasource, e.g. `tsdb` or `sql`
+    def category
+      @model['meta']['category']
+    end
+
+    # @return [String] type of the datasource, e.g. `mysql`
+    def type
+      @model['type'] || @model['meta']['id']
+    end
+
     # @return [String] name of the datasource
     def name
       @model['name']
diff --git a/lib/grafana/errors.rb b/lib/grafana/errors.rb
index 9b50680..72f2adc 100644
--- a/lib/grafana/errors.rb
+++ b/lib/grafana/errors.rb
@@ -57,21 +57,13 @@ def initialize(panel)
     end
   end
 
-  # Raised if no SQL query is specified in a {AbstractSqlQuery} object.
+  # Raised if no SQL query is specified.
   class MissingSqlQueryError < GrafanaError
     def initialize
       super('No SQL statement has been specified.')
     end
   end
 
-  # Raised if a datasource shall be queried, which is not (yet) supported by the reporter
-  class DatasourceTypeNotSupportedError < GrafanaError
-    def initialize(name, type)
-      super("The configured datasource with name '#{name}' is of type '#{type}', which is currently "\
-            'not supported by ruby-grafana-reporter. It will only be usable in panel image queries.')
-    end
-  end
-
   # Raised if a datasource shall be queried, which is not (yet) supported by the reporter
   class InvalidDatasourceQueryProvidedError < GrafanaError
     def initialize(query)
diff --git a/lib/grafana/grafana.rb b/lib/grafana/grafana.rb
index d74b190..b5037ea 100644
--- a/lib/grafana/grafana.rb
+++ b/lib/grafana/grafana.rb
@@ -9,6 +9,8 @@
 module Grafana
   # Main class for handling the interaction with one specific Grafana instance.
   class Grafana
+    attr_reader :logger
+
     # @param base_uri [String] full URI pointing to the specific grafana instance without
     #   trailing slash, e.g. +https://localhost:3000+.
     # @param key [String] API key for the grafana instance, if required
@@ -116,13 +118,7 @@ def initialize_datasources
 
       json = JSON.parse(settings.body)
       json['datasources'].select { |_k, v| v['id'].to_i.positive? }.each do |ds_name, ds_value|
-        begin
-          @datasources[ds_name] = AbstractDatasource.build_instance(ds_value)
-        rescue DatasourceTypeNotSupportedError => e
-          # an unsupported datasource type has been configured in the dashboard
-          # - no worries here
-          @logger.warn(e.message)
-        end
+        @datasources[ds_name] = AbstractDatasource.build_instance(ds_value)
       end
       @datasources['default'] = @datasources[json['defaultDatasource']]
     end
diff --git a/lib/grafana/graphite_datasource.rb b/lib/grafana/graphite_datasource.rb
index bd193ae..70e60ea 100644
--- a/lib/grafana/graphite_datasource.rb
+++ b/lib/grafana/graphite_datasource.rb
@@ -3,6 +3,12 @@
 module Grafana
   # Implements the interface to graphite datasources.
   class GraphiteDatasource < AbstractDatasource
+    # @see AbstractDatasource#handles?
+    def self.handles?(model)
+      tmp = new(model)
+      tmp.type == 'graphite'
+    end
+
     # +:raw_query+ needs to contain a Graphite query as String
     # @see AbstractDatasource#request
     def request(query_description)
diff --git a/lib/grafana/influxdb_datasource.1disabled_rb b/lib/grafana/influxdb_datasource.1disabled_rb
deleted file mode 100644
index d9e3da2..0000000
--- a/lib/grafana/influxdb_datasource.1disabled_rb
+++ /dev/null
@@ -1,31 +0,0 @@
-# frozen_string_literal: true
-
-module Grafana
-  class InfluxDbDatasource < AbstractDatasource
-
-    def initialize(ds_model)
-      @model = ds_model
-    end
-
-    def model
-      @model
-    end
-
-    def url(query)
-      raise NotImplementedError
-      "/api/datasources/proxy/#{model.id}/query"
-      # db
-      # q includes times somehow...
-    end
-
-    def request(query)
-      {
-        request: Net::HTTP::Get
-      }
-    end
-
-    def raw_query(target)
-      target['query']
-    end
-  end
-end
diff --git a/lib/grafana/prometheus_datasource.rb b/lib/grafana/prometheus_datasource.rb
index f2020a2..bdddd56 100644
--- a/lib/grafana/prometheus_datasource.rb
+++ b/lib/grafana/prometheus_datasource.rb
@@ -3,6 +3,12 @@
 module Grafana
   # Implements the interface to Prometheus datasources.
   class PrometheusDatasource < AbstractDatasource
+    # @see AbstractDatasource#handles?
+    def self.handles?(model)
+      tmp = new(model)
+      tmp.type == 'prometheus'
+    end
+
     # +:raw_query+ needs to contain a Prometheus query as String
     # @see AbstractDatasource#request
     def request(query_description)
diff --git a/lib/grafana/sql_datasource.rb b/lib/grafana/sql_datasource.rb
index 097cec7..582ffc8 100644
--- a/lib/grafana/sql_datasource.rb
+++ b/lib/grafana/sql_datasource.rb
@@ -3,6 +3,12 @@
 module Grafana
   # Implements the interface to all SQL based datasources (tested with PostgreSQL and MariaDB/MySQL).
   class SqlDatasource < AbstractDatasource
+    # @see AbstractDatasource#handles?
+    def self.handles?(model)
+      tmp = new(model)
+      tmp.category == 'sql'
+    end
+
     # +:raw_query+ needs to contain a SQL query as String in the respective database dialect
     # @see AbstractDatasource#request
     def request(query_description)
diff --git a/lib/grafana/unsupported_datasource.rb b/lib/grafana/unsupported_datasource.rb
new file mode 100644
index 0000000..44888b9
--- /dev/null
+++ b/lib/grafana/unsupported_datasource.rb
@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+module Grafana
+  # Dummy class, which is used, if a datasource is currently unsupported.
+  class UnsupportedDatasource < AbstractDatasource
+  end
+end
diff --git a/lib/grafana/webrequest.rb b/lib/grafana/webrequest.rb
index 9787df5..1322344 100644
--- a/lib/grafana/webrequest.rb
+++ b/lib/grafana/webrequest.rb
@@ -59,7 +59,7 @@ def execute(timeout = nil)
     def configure_ssl
       @http.use_ssl = true
       @http.verify_mode = OpenSSL::SSL::VERIFY_PEER
-      if self.class.ssl_cert && !File.exist?(self.class.ssl_cert)
+      if self.class.ssl_cert && !File.file?(self.class.ssl_cert)
         @logger.warn('SSL certificate file does not exist.')
       elsif self.class.ssl_cert
         @http.cert_store = OpenSSL::X509::Store.new
diff --git a/lib/grafana_reporter/abstract_query.rb b/lib/grafana_reporter/abstract_query.rb
index ece0eec..9f9c13a 100644
--- a/lib/grafana_reporter/abstract_query.rb
+++ b/lib/grafana_reporter/abstract_query.rb
@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 module GrafanaReporter
-  # @abstract Override {#pre_process}, {#post_process} and {#self.build_demo_entry} in subclass.
+  # @abstract Override {#pre_process} and {#post_process} in subclass.
   #
   # Superclass containing everything for all queries towards grafana.
   class AbstractQuery
@@ -9,7 +9,7 @@ class AbstractQuery
     attr_writer :raw_query
     attr_reader :variables, :result, :panel
 
-    # @param grafana_or_panel [Object] {Grafana} or {Panel} object for which the query is executed
+    # @param grafana_or_panel [Object] {Grafana::Grafana} or {Grafana::Panel} object for which the query is executed
     def initialize(grafana_or_panel)
       if grafana_or_panel.is_a?(Grafana::Panel)
         @panel = grafana_or_panel
@@ -24,7 +24,7 @@ def initialize(grafana_or_panel)
     #
     # Runs the whole process to receive values properly from this query:
     # - calls {#pre_process}
-    # - executes this query against the {Grafana} instance
+    # - executes this query against the {Grafana::AbstractDatasource} implementation instance
     # - calls {#post_process}
     #
     # @return [Hash] result of the query in standardized format
@@ -32,25 +32,14 @@ def execute
       return @result unless @result.nil?
 
       pre_process
+      raise DatasourceNotSupportedError.new(@datasource, self) if @datasource.is_a?(Grafana::UnsupportedDatasource)
+
       @result = @datasource.request(from: @from, to: @to, raw_query: raw_query, variables: grafana_variables,
                                     prepared_request: @grafana.prepare_request, timeout: timeout)
       post_process
       @result
     end
 
-    # Sets default configurations from the given {Dashboard} and store them as settings in the query.
-    #
-    # Following data is extracted:
-    # - +from+, by {Dashboard#from_time}
-    # - +to+, by {Dashboard#to_time}
-    # - and all variables as {Variable}, prefixed with +var-+, as grafana also does it
-    # @param dashboard [Dashboard] dashboard from which the defaults are captured
-    def set_defaults_from_dashboard(dashboard)
-      @from = dashboard.from_time
-      @to = dashboard.to_time
-      dashboard.variables.each { |item| merge_variables({ "var-#{item.name}": item }) }
-    end
-
     # Overwrite this function to extract a proper raw query value from this object.
     #
     # If the property +@raw_query+ is not set manually by the calling object, this
@@ -77,35 +66,22 @@ def post_process
       raise NotImplementedError
     end
 
-    # Merges the given hashes to the current object by using the {#merge_variables} method.
-    # It respects the priorities of the hashes and the object and allows only valid variables to be passed.
-    # @param document_hash [Hash] variables from report template level
-    # @param item_hash [Hash] variables from item configuration level, i.e. specific call, which may override document
-    # @return [void]
-    # TODO: rename method
-    def merge_hash_variables(document_hash, item_hash)
-      sel_doc_items = document_hash.select do |k, _v|
-        k =~ /^var-/ || k == 'grafana-report-timestamp' || k =~ /grafana_default_(?:from|to)_timezone/
-      end
-      merge_variables(sel_doc_items.each_with_object({}) { |(k, v), h| h[k] = ::Grafana::Variable.new(v) })
-
-      sel_items = item_hash.select do |k, _v|
-        # TODO: specify accepted options in each class or check if simply all can be allowed with prefix +var-+
-        k =~ /^var-/ || k =~ /^render-/ || k =~ /filter_columns|format|replace_values_.*|transpose|column_divider|
-                                                 row_divider|from_timezone|to_timezone|result_type|query/x
-      end
-      merge_variables(sel_items.each_with_object({}) { |(k, v), h| h[k] = ::Grafana::Variable.new(v) })
+    # Used to specify variables to be used for this query. This method ensures, that only the values of the
+    # {Grafana::Variable} stored in the +variables+ Array are overwritten.
+    # @param name [String] name of the variable to set
+    # @param variable [Grafana::Variable] variable from which the {Grafana::Variable#raw_value} will be assigned to the query variables
+    def assign_variable(name, variable)
+      raise GrafanaReporterError, "Provided variable is not of type Grafana::Variable (name: '#{name}', value: '#{value}')" unless variable.is_a?(Grafana::Variable)
 
-      @timeout = item_hash['timeout'] || document_hash['grafana-default-timeout'] || @timeout
-      @from = item_hash['from'] || document_hash['from'] || @from
-      @to = item_hash['to'] || document_hash['to'] || @to
+      @variables[name] ||= variable
+      @variables[name].raw_value = variable.raw_value
     end
 
     # Transposes the given result.
     #
     # NOTE: Only the +:content+ of the given result hash is transposed. The +:header+ is ignored.
     #
-    # @param result [Hash] preformatted sql hash, (see {Grafana::AbstractDatasource#preformat_response})
+    # @param result [Hash] preformatted sql hash, (see {Grafana::AbstractDatasource#request})
     # @param transpose_variable [Grafana::Variable] true, if the result hash shall be transposed
     # @return [Hash] transposed query result
     def transpose(result, transpose_variable)
@@ -121,7 +97,7 @@ def transpose(result, transpose_variable)
     #
     # Multiple columns may be filtered. Therefore the column titles have to be named in the
     # {Grafana::Variable#raw_value} and have to be separated by +,+ (comma).
-    # @param result [Hash] preformatted sql hash, (see {Grafana::AbstractDatasource#preformat_response})
+    # @param result [Hash] preformatted sql hash, (see {Grafana::AbstractDatasource#request})
     # @param filter_columns_variable [Grafana::Variable] column names, which shall be removed in the query result
     # @return [Hash] filtered query result
     def filter_columns(result, filter_columns_variable)
@@ -145,10 +121,9 @@ def filter_columns(result, filter_columns_variable)
     # The formatting will be applied separately for every column. Therefore the column formats have to be named
     # in the {Grafana::Variable#raw_value} and have to be separated by +,+ (comma). If no value is specified for
     # a column, no change will happen.
-    # @param result [Hash] preformatted sql hash, (see {Grafana::AbstractDatasource#preformat_response})
+    # @param result [Hash] preformatted sql hash, (see {Grafana::AbstractDatasource#request})
     # @param formats [Grafana::Variable] formats, which shall be applied to the columns in the query result
     # @return [Hash] formatted query result
-    # TODO: make sure that caught errors are also visible in logger
     def format_columns(result, formats)
       return result unless formats
 
@@ -162,6 +137,7 @@ def format_columns(result, formats)
           begin
             row[i] = format % row[i] if row[i]
           rescue StandardError => e
+            @grafana.logger.error(e.message)
             row[i] = e.message
           end
         end
@@ -195,7 +171,7 @@ def format_columns(result, formats)
     # '42 is the answer'. Important to know: the regular expressions always have to start
     # with +^+ and end with +$+, i.e. the expression itself always has to match
     # the whole content in one field.
-    # @param result [Hash] preformatted query result (see {Grafana::AbstractDatasource#preformat_response}.
+    # @param result [Hash] preformatted query result (see {Grafana::AbstractDatasource#request}.
     # @param configs [Array<Grafana::Variable>] one variable for replacing values in one column
     # @return [Hash] query result with replaced values
     # TODO: make sure that caught errors are also visible in logger
@@ -280,7 +256,8 @@ def replace_values(result, configs)
     # @param timezone [Grafana::Variable] timezone to use, if not system timezone
     # @return [String] translated date as timestamp string
     def translate_date(orig_date, report_time, is_to_time, timezone = nil)
-      report_time ||= Variable.new(Time.now.to_s)
+      # TODO: add test case for creation of variable, if not given, maybe also print a warning
+      report_time ||= ::Grafana::Variable.new(Time.now.to_s)
       return (DateTime.parse(report_time.raw_value).to_time.to_i * 1000).to_s unless orig_date
       return orig_date if orig_date =~ /^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}\.\d{3}Z$/
       return orig_date if orig_date =~ /^\d+$/
@@ -378,24 +355,5 @@ def fit_date(date, fit_letter, is_to_time)
 
       date
     end
-
-    # Merges the given Hash with the stored variables.
-    #
-    # Can be used to easily set many values at once in the local variables hash.
-    #
-    # Please note, that the values of the Hash need to be of type {Variable}.
-    #
-    # @param hash [Hash<String,Variable>] Hash containing variable name as key and {Variable} as value
-    # @return [AbstractQuery] this object
-    # TODO: test if this method can be removed, or make it private at least
-    def merge_variables(hash)
-      hash.each do |k, v|
-        if @variables[k.to_s].nil?
-          @variables[k.to_s] = v
-        else
-          @variables[k.to_s].raw_value = v.raw_value
-        end
-      end
-    end
   end
 end
diff --git a/lib/grafana_reporter/abstract_report.rb b/lib/grafana_reporter/abstract_report.rb
index 118f9df..7a069a7 100644
--- a/lib/grafana_reporter/abstract_report.rb
+++ b/lib/grafana_reporter/abstract_report.rb
@@ -35,21 +35,13 @@ class AbstractReport
     attr_reader :done
 
     # @param config [Configuration] configuration object
-    # @param template [String] path to the template to be used
-    # @param destination_file_or_path [String or File] path to the destination report or file object to use
-    # @param custom_attributes [Hash] custom attributes, which shall be merged with priority over the configuration
-    def initialize(config, template, destination_file_or_path = nil, custom_attributes = {})
+    def initialize(config)
       @config = config
       @logger = Logger::TwoWayDelegateLogger.new
       @logger.additional_logger = @config.logger
-      @done = false
-      @template = template
-      @destination_file_or_path = destination_file_or_path
-      @custom_attributes = custom_attributes
-      @start_time = nil
-      @end_time = nil
-      @cancel = false
-      raise MissingTemplateError, @template.to_s unless File.exist?(@template.to_s)
+      @grafana_instances = {}
+
+      init_before_create
     end
 
     # Registers a new event listener object.
@@ -66,6 +58,17 @@ def self.clear_event_listeners
       @@event_listeners.default = []
     end
 
+    # @param instance [String] requested grafana instance
+    # @return [Grafana::Grafana] the requested grafana instance.
+    def grafana(instance)
+      unless @grafana_instances[instance]
+        @grafana_instances[instance] = ::Grafana::Grafana.new(@config.grafana_host(instance),
+                                                              @config.grafana_api_key(instance),
+                                                              logger: @logger)
+      end
+      @grafana_instances[instance]
+    end
+
     # Call to request cancelling the report generation.
     # @return [void]
     def cancel!
@@ -122,25 +125,73 @@ def full_log
     end
 
     # Is being called to start the report generation.
+    # @param template [String] path to the template to be used, trailing +.adoc+ extension may be omitted
+    # @param destination_file_or_path [String or File] path to the destination report or file object to use
+    # @param custom_attributes [Hash] custom attributes, which shall be merged with priority over the configuration
     # @return [void]
-    def create_report
+    def create_report(template, destination_file_or_path = nil, custom_attributes = {})
+      init_before_create
+      @template = template
+      @destination_file_or_path = destination_file_or_path
+      @custom_attributes = custom_attributes
+
+      # automatically add extension, if a file with adoc extension exists
+      @template = "#{@template}.adoc" if File.file?("#{@template}.adoc") && !File.file?(@template.to_s)
+      raise MissingTemplateError, @template.to_s unless File.file?(@template.to_s)
+
       notify(:on_before_create)
       @start_time = Time.new
       logger.info("Report started at #{@start_time}")
     end
 
-    # @abstract
+    # Used to calculate the progress of a report. By default expects +@total_steps+ to contain the total
+    # number of steps, which will be processed with each call of {#next_step}.
     # @return [Integer] number between 0 and 100, representing the current progress of the report creation.
     def progress
+      return @current_pos.to_i if @total_steps.to_i.zero?
+
+      @current_pos.to_f / @total_steps
+    end
+
+    # Increments the progress.
+    # @return [Integer] number of the current progress position.
+    def next_step
+      @current_pos += 1
+      @current_pos
+    end
+
+    # @abstract
+    # Provided class objects need to implement a method +build_demo_entry(panel)+.
+    # @return [Array<Class>] array of class objects, which shall be included in a demo report
+    def self.demo_report_classes
       raise NotImplementedError
     end
 
     private
 
+    # Called, if the report generation has died with an error.
+    # @param error [StandardError] occured error
+    # @return [void]
+    def died_with_error(error)
+      @error = [error.message] << [error.backtrace]
+      done!
+    end
+
+    def init_before_create
+      @done = false
+      @start_time = nil
+      @end_time = nil
+      @cancel = false
+      @current_pos = 0
+    end
+
     def done!
+      return if @done
+
       @done = true
       @end_time = Time.new
-      logger.info("Report creation ended after #{@end_time - @start_time} seconds with status '#{status}'")
+      @start_time ||= @end_time
+      logger.info("Report creation ended after #{@end_time.to_i - @start_time.to_i} seconds with status '#{status}'")
       notify(:on_after_finish)
     end
 
diff --git a/lib/grafana_reporter/alerts_table_query.rb b/lib/grafana_reporter/alerts_table_query.rb
index fb53471..122b139 100644
--- a/lib/grafana_reporter/alerts_table_query.rb
+++ b/lib/grafana_reporter/alerts_table_query.rb
@@ -19,18 +19,18 @@ class AlertsTableQuery < AbstractQuery
     def pre_process
       raise MissingMandatoryAttributeError, 'columns' unless @raw_query['columns']
 
-      @from = translate_date(@from, @variables['grafana-report-timestamp'], false, @variables['from_timezone'] ||
+      @from = translate_date(@from, @variables['grafana_report_timestamp'], false, @variables['from_timezone'] ||
                              @variables['grafana_default_from_timezone'])
-      @to = translate_date(@to, @variables['grafana-report-timestamp'], true, @variables['to_timezone'] ||
+      @to = translate_date(@to, @variables['grafana_report_timestamp'], true, @variables['to_timezone'] ||
                            @variables['grafana_default_to_timezone'])
       @datasource = Grafana::GrafanaAlertsDatasource.new(nil)
     end
 
     # Filter the query result for the given columns and sets the result in the preformatted SQL
     # result stlye.
-
-    # Additionally it applies {QueryMixin#format_columns}, {QueryMixin#replace_values} and
-    # {QueryMixin#filter_columns}.
+    #
+    # Additionally it applies {AbstractQuery#format_columns}, {AbstractQuery#replace_values} and
+    # {AbstractQuery#filter_columns}.
     # @return [void]
     def post_process
       @result = format_columns(@result, @variables['format'])
diff --git a/lib/grafana_reporter/annotations_table_query.rb b/lib/grafana_reporter/annotations_table_query.rb
index 4485ffd..04993b6 100644
--- a/lib/grafana_reporter/annotations_table_query.rb
+++ b/lib/grafana_reporter/annotations_table_query.rb
@@ -18,9 +18,9 @@ class AnnotationsTableQuery < AbstractQuery
     def pre_process
       raise MissingMandatoryAttributeError, 'columns' unless @raw_query['columns']
 
-      @from = translate_date(@from, @variables['grafana-report-timestamp'], false, @variables['from_timezone'] ||
+      @from = translate_date(@from, @variables['grafana_report_timestamp'], false, @variables['from_timezone'] ||
                              @variables['grafana_default_from_timezone'])
-      @to = translate_date(@to, @variables['grafana-report-timestamp'], true, @variables['to_timezone'] ||
+      @to = translate_date(@to, @variables['grafana_report_timestamp'], true, @variables['to_timezone'] ||
                            @variables['grafana_default_to_timezone'])
       @datasource = Grafana::GrafanaAnnotationsDatasource.new(nil)
     end
@@ -28,8 +28,8 @@ def pre_process
     # Filters the query result for the given columns and sets the result
     # in the preformatted SQL result style.
     #
-    # Additionally it applies {QueryMixin#format_columns}, {QueryMixin#replace_values} and
-    # {QueryMixin#filter_columns}.
+    # Additionally it applies {AbstractQuery#format_columns}, {AbstractQuery#replace_values} and
+    # {AbstractQuery#filter_columns}.
     # @return [void]
     def post_process
       @result = format_columns(@result, @variables['format'])
diff --git a/lib/grafana_reporter/application/application.rb b/lib/grafana_reporter/application/application.rb
index 11725dd..199de0b 100644
--- a/lib/grafana_reporter/application/application.rb
+++ b/lib/grafana_reporter/application/application.rb
@@ -13,8 +13,6 @@ module Application
     # It can be run to test the grafana connection, render a single template
     # or run as a service.
     class Application
-      # Default file name for grafana reporter configuration file
-      CONFIG_FILE = 'grafana_reporter.config'
 
       # Contains the {Configuration} object of the application.
       attr_accessor :config
@@ -32,7 +30,7 @@ def initialize
       # @param params [Array<String>] command line parameters, mainly ARGV can be used.
       # @return [Integer] 0 if everything is fine, -1 if execution aborted.
       def configure_and_run(params = [])
-        config_file = CONFIG_FILE
+        config_file = Configuration::DEFAULT_CONFIG_FILE_NAME
         tmp_config = Configuration.new
         action_wizard = false
 
@@ -44,10 +42,14 @@ def configure_and_run(params = [])
                         end
 
           opts.on('-c', '--config CONFIG_FILE_NAME', 'Specify custom configuration file,'\
-                  " instead of #{CONFIG_FILE}.") do |file_name|
+                  " instead of #{Configuration::DEFAULT_CONFIG_FILE_NAME}.") do |file_name|
             config_file = file_name
           end
 
+          opts.on('-r', '--register FILE', 'Register a custom plugin, e.g. your own Datasource implementation') do |plugin|
+            require plugin
+          end
+
           opts.on('-d', '--debug LEVEL', 'Specify detail level: FATAL, ERROR, WARN, INFO, DEBUG.') do |level|
             tmp_config.set_param('grafana-reporter:debug-level', level)
           end
@@ -65,7 +67,7 @@ def configure_and_run(params = [])
 
           opts.on('--ssl-cert FILE', 'Manually specify a SSL cert file for HTTPS connection to grafana. Only '\
                   'needed if not working properly otherwise.') do |file|
-            if File.exist?(file)
+            if File.file?(file)
               tmp_config.set_param('grafana-reporter:ssl-cert', file)
             else
               config.logger.warn("SSL certificate file #{file} does not exist. Setting will be ignored.")
@@ -106,22 +108,14 @@ def configure_and_run(params = [])
         end
 
         # abort if config file does not exist
-        unless File.exist?(config_file)
+        unless File.file?(config_file)
           puts "Config file '#{config_file}' does not exist. Consider calling the configuration wizard"\
                ' with option \'-w\' or use \'-h\' to see help message. Aborting.'
           return -1
         end
 
-        # read config file
-        config_hash = nil
-        begin
-          config_hash = YAML.load_file(config_file)
-        rescue StandardError => e
-          raise ConfigurationError, "Could not read config file '#{config_file}' (Error: #{e.message})"
-        end
-
         # merge command line configuration with read config file
-        @config.config = config_hash
+        @config.load_config_from_file(config_file)
         @config.merge!(tmp_config)
 
         run
@@ -146,7 +140,7 @@ def run
 
         when Configuration::MODE_SINGLE_RENDER
           begin
-            config.report_class.new(config, config.template, config.to_file).create_report
+            config.report_class.new(config).create_report(config.template, config.to_file)
           rescue StandardError => e
             puts "#{e.message}\n#{e.backtrace.join("\n")}"
           end
diff --git a/lib/grafana_reporter/application/webservice.rb b/lib/grafana_reporter/application/webservice.rb
index 186c492..65655a9 100644
--- a/lib/grafana_reporter/application/webservice.rb
+++ b/lib/grafana_reporter/application/webservice.rb
@@ -6,9 +6,12 @@ module Application
     # make use of `webrick` or similar, so that it can be used without futher dependencies
     # in conjunction with the standard asciidoctor docker container.
     class Webservice
+      # Array of possible webservice running states
+      STATUS = %I[stopped running stopping].freeze
+
       def initialize
         @reports = []
-        @running = false
+        @status = :stopped
       end
 
       # Runs the webservice with the given {Configuration} object.
@@ -19,17 +22,32 @@ def run(config)
         # start webserver
         @server = TCPServer.new(@config.webserver_port)
         @logger.info("Server listening on port #{@config.webserver_port}...")
-        @running = true
 
         @progress_reporter = Thread.new {}
 
+        @status = :running
         accept_requests_loop
-        @running = false
+        @status = :stopped
+      end
+
+      # @return True, if webservice is stopped, false otherwise
+      def stopped?
+        @status == :stopped
       end
 
       # @return True, if webservice is up and running, false otherwise
       def running?
-        @running
+        @status == :running
+      end
+
+      # Forces stopping the webservice.
+      def stop!
+        @status = :stopping
+
+        # invoke a new request, so that the webservice stops.
+        socket = TCPSocket.new('localhost', @config.webserver_port)
+        socket.send '', 0
+        socket.close
       end
 
       private
@@ -39,6 +57,14 @@ def accept_requests_loop
           # step 1) accept incoming connection
           socket = @server.accept
 
+          # TODO: shutdown properly on SIGINT/SIGHUB
+
+          # stop webservice properly, if shall be shutdown
+          if @status == :stopping
+            socket.close
+            break
+          end
+
           # step 2) print the request headers (separated by a blank line e.g. \r\n)
           request = ''
           line = ''
@@ -57,9 +83,6 @@ def accept_requests_loop
           rescue WebserviceUnknownPathError => e
             @logger.debug(e.message)
             socket.write http_response(404, '', e.message)
-          rescue MissingTemplateError => e
-            @logger.error(e.message)
-            socket.write http_response(400, 'Bad Request', e.message)
           rescue WebserviceGeneralRenderingError => e
             @logger.fatal(e.message)
             socket.write http_response(400, 'Bad Request', e.message)
@@ -179,7 +202,7 @@ def view_report(attrs)
 
       def render_report(attrs)
         # build report
-        template_file = "#{@config.templates_folder}#{attrs['var-template']}.adoc"
+        template_file = "#{@config.templates_folder}#{attrs['var-template']}"
 
         file = Tempfile.new('gf_pdf_', @config.reports_folder)
         begin
@@ -188,9 +211,10 @@ def render_report(attrs)
           @logger.debug("File permissions could not be set for #{file.path}: #{e.message}")
         end
 
-        report = @config.report_class.new(@config, template_file, file, attrs)
+        report = @config.report_class.new(@config)
+        Thread.report_on_exception = false
         Thread.new do
-          report.create_report
+          report.create_report(template_file, file, attrs)
         end
         @reports << report
 
diff --git a/lib/grafana_reporter/asciidoctor/alerts_table_include_processor.rb b/lib/grafana_reporter/asciidoctor/alerts_table_include_processor.rb
index 4811394..afac36c 100644
--- a/lib/grafana_reporter/asciidoctor/alerts_table_include_processor.rb
+++ b/lib/grafana_reporter/asciidoctor/alerts_table_include_processor.rb
@@ -31,11 +31,11 @@ module Asciidoctor
     #
     # +to+ - 'to' time for the sql query
     #
-    # +format+ - see {QueryMixin#format_columns}
+    # +format+ - see {AbstractQuery#format_columns}
     #
-    # +replace_values+ - see {QueryMixin#replace_values}
+    # +replace_values+ - see {AbstractQuery#replace_values}
     #
-    # +filter_columns+ - see {QueryMixin#filter_columns}
+    # +filter_columns+ - see {AbstractQuery#filter_columns}
     class AlertsTableIncludeProcessor < ::Asciidoctor::Extensions::IncludeProcessor
       include ProcessorMixin
 
@@ -56,12 +56,12 @@ def process(doc, reader, _target, attrs)
                              " dashboard: #{dashboard_id}, panel: #{panel_id})")
 
         query = AlertsTableQuery.new(@report.grafana(instance))
-        query.set_defaults_from_dashboard(@report.grafana(instance).dashboard(dashboard_id)) if dashboard_id
+        assign_dashboard_defaults(query, @report.grafana(instance).dashboard(dashboard_id)) if dashboard_id
         defaults = {}
         defaults['dashboardId'] = dashboard_id if dashboard_id
         defaults['panelId'] = panel_id if panel_id
 
-        query.merge_hash_variables(doc.attributes, attrs)
+        assign_doc_and_item_variables(query, doc.attributes, attrs)
         selected_attrs = attrs.select do |k, _v|
           k =~ /(?:columns|limit|folderId|dashboardId|panelId|dahboardTag|dashboardQuery|state|query)/x
         end
diff --git a/lib/grafana_reporter/asciidoctor/annotations_table_include_processor.rb b/lib/grafana_reporter/asciidoctor/annotations_table_include_processor.rb
index b97675b..2d1babc 100644
--- a/lib/grafana_reporter/asciidoctor/annotations_table_include_processor.rb
+++ b/lib/grafana_reporter/asciidoctor/annotations_table_include_processor.rb
@@ -31,11 +31,11 @@ module Asciidoctor
     #
     # +to+ - 'to' time for the sql query
     #
-    # +format+ - see {QueryMixin#format_columns}
+    # +format+ - see {AbstractQuery#format_columns}
     #
-    # +replace_values+ - see {QueryMixin#replace_values}
+    # +replace_values+ - see {AbstractQuery#replace_values}
     #
-    # +filter_columns+ - see {QueryMixin#filter_columns}
+    # +filter_columns+ - see {AbstractQuery#filter_columns}
     class AnnotationsTableIncludeProcessor < ::Asciidoctor::Extensions::IncludeProcessor
       include ProcessorMixin
 
@@ -55,12 +55,12 @@ def process(doc, reader, _target, attrs)
         @report.logger.debug("Processing AnnotationsTableIncludeProcessor (instance: #{instance})")
 
         query = AnnotationsTableQuery.new(@report.grafana(instance))
-        query.set_defaults_from_dashboard(@report.grafana(instance).dashboard(dashboard_id)) if dashboard_id
+        assign_dashboard_defaults(query, @report.grafana(instance).dashboard(dashboard_id)) if dashboard_id
         defaults = {}
         defaults['dashboardId'] = dashboard_id if dashboard_id
         defaults['panelId'] = panel_id if panel_id
 
-        query.merge_hash_variables(doc.attributes, attrs)
+        assign_doc_and_item_variables(query, doc.attributes, attrs)
         selected_attrs = attrs.select do |k, _v|
           k =~ /(?:columns|limit|alertId|dashboardId|panelId|userId|type|tags)/
         end
diff --git a/lib/grafana_reporter/asciidoctor/panel_image_block_macro.rb b/lib/grafana_reporter/asciidoctor/panel_image_block_macro.rb
index 77a908c..af45a82 100644
--- a/lib/grafana_reporter/asciidoctor/panel_image_block_macro.rb
+++ b/lib/grafana_reporter/asciidoctor/panel_image_block_macro.rb
@@ -47,7 +47,8 @@ def process(parent, target, attrs)
 
         begin
           query = PanelImageQuery.new(@report.grafana(instance).dashboard(dashboard).panel(target))
-          query.merge_hash_variables(parent.document.attributes, attrs)
+          assign_dashboard_defaults(query, @report.grafana(instance).dashboard(dashboard))
+          assign_doc_and_item_variables(query, parent.document.attributes, attrs)
           @report.logger.debug("from: #{query.from}, to: #{query.to}")
 
           image = query.execute
diff --git a/lib/grafana_reporter/asciidoctor/panel_image_inline_macro.rb b/lib/grafana_reporter/asciidoctor/panel_image_inline_macro.rb
index a45e292..cc1a59b 100644
--- a/lib/grafana_reporter/asciidoctor/panel_image_inline_macro.rb
+++ b/lib/grafana_reporter/asciidoctor/panel_image_inline_macro.rb
@@ -44,13 +44,15 @@ def process(parent, target, attrs)
         dashboard = attrs['dashboard'] || parent.document.attr('grafana_default_dashboard')
         @report.logger.debug("Processing PanelImageInlineMacro (instance: #{instance}, dashboard: #{dashboard},"\
                              " panel: #{target})")
-        query = PanelImageQuery.new(@report.grafana(instance).dashboard(dashboard).panel(target))
-        # set alt text to a default, because otherwise asciidoctor fails
-        attrs['alt'] = '' unless attrs['alt']
-        query.merge_hash_variables(parent.document.attributes, attrs)
-        @report.logger.debug("from: #{query.from}, to: #{query.to}")
 
         begin
+          query = PanelImageQuery.new(@report.grafana(instance).dashboard(dashboard).panel(target))
+          # set alt text to a default, because otherwise asciidoctor fails
+          attrs['alt'] = '' unless attrs['alt']
+          assign_dashboard_defaults(query, @report.grafana(instance).dashboard(dashboard))
+          assign_doc_and_item_variables(query, parent.document.attributes, attrs)
+          @report.logger.debug("from: #{query.from}, to: #{query.to}")
+
           image = query.execute
           image_path = @report.save_image_file(image)
         rescue GrafanaReporterError => e
diff --git a/lib/grafana_reporter/asciidoctor/panel_property_inline_macro.rb b/lib/grafana_reporter/asciidoctor/panel_property_inline_macro.rb
index 4977797..f86c2c5 100644
--- a/lib/grafana_reporter/asciidoctor/panel_property_inline_macro.rb
+++ b/lib/grafana_reporter/asciidoctor/panel_property_inline_macro.rb
@@ -40,7 +40,8 @@ def process(parent, target, attrs)
         begin
           query = PanelPropertyQuery.new(@report.grafana(instance).dashboard(dashboard).panel(target))
           query.raw_query = { property_name: attrs[:field] }
-          query.merge_hash_variables(parent.document.attributes, attrs)
+          assign_dashboard_defaults(query, @report.grafana(instance).dashboard(dashboard))
+          assign_doc_and_item_variables(query, parent.document.attributes, attrs)
           @report.logger.debug("from: #{query.from}, to: #{query.to}")
 
           description = query.execute
diff --git a/lib/grafana_reporter/asciidoctor/panel_query_table_include_processor.rb b/lib/grafana_reporter/asciidoctor/panel_query_table_include_processor.rb
index 39e5f17..cc4c394 100644
--- a/lib/grafana_reporter/asciidoctor/panel_query_table_include_processor.rb
+++ b/lib/grafana_reporter/asciidoctor/panel_query_table_include_processor.rb
@@ -32,11 +32,11 @@ module Asciidoctor
     #
     # +to+ - 'to' time for the sql query
     #
-    # +format+ - see {QueryMixin#format_columns}
+    # +format+ - see {AbstractQuery#format_columns}
     #
-    # +replace_values+ - see {QueryMixin#replace_values}
+    # +replace_values+ - see {AbstractQuery#replace_values}
     #
-    # +filter_columns+ - see {QueryMixin#filter_columns}
+    # +filter_columns+ - see {AbstractQuery#filter_columns}
     class PanelQueryTableIncludeProcessor < ::Asciidoctor::Extensions::IncludeProcessor
       include ProcessorMixin
 
@@ -60,8 +60,8 @@ def process(doc, reader, target, attrs)
         begin
           panel = @report.grafana(instance).dashboard(dashboard).panel(panel_id)
           query = QueryValueQuery.new(panel)
-          query.set_defaults_from_dashboard(panel.dashboard)
-          query.merge_hash_variables(doc.attributes, attrs)
+          assign_dashboard_defaults(query, panel.dashboard)
+          assign_doc_and_item_variables(query, doc.attributes, attrs)
           @report.logger.debug("from: #{query.from}, to: #{query.to}")
 
           reader.unshift_lines query.execute
diff --git a/lib/grafana_reporter/asciidoctor/panel_query_value_inline_macro.rb b/lib/grafana_reporter/asciidoctor/panel_query_value_inline_macro.rb
index 3c46b18..17f1afa 100644
--- a/lib/grafana_reporter/asciidoctor/panel_query_value_inline_macro.rb
+++ b/lib/grafana_reporter/asciidoctor/panel_query_value_inline_macro.rb
@@ -32,11 +32,11 @@ module Asciidoctor
     #
     # +to+ - 'to' time for the sql query
     #
-    # +format+ - see {QueryMixin#format_columns}
+    # +format+ - see {AbstractQuery#format_columns}
     #
-    # +replace_values+ - see {QueryMixin#replace_values}
+    # +replace_values+ - see {AbstractQuery#replace_values}
     #
-    # +filter_columns+ - see {QueryMixin#filter_columns}
+    # +filter_columns+ - see {AbstractQuery#filter_columns}
     class PanelQueryValueInlineMacro < ::Asciidoctor::Extensions::InlineMacroProcessor
       include ProcessorMixin
       use_dsl
@@ -57,8 +57,8 @@ def process(parent, target, attrs)
         begin
           panel = @report.grafana(instance).dashboard(dashboard).panel(target)
           query = QueryValueQuery.new(panel)
-          query.set_defaults_from_dashboard(panel.dashboard)
-          query.merge_hash_variables(parent.document.attributes, attrs)
+          assign_dashboard_defaults(query, panel.dashboard)
+          assign_doc_and_item_variables(query, parent.document.attributes, attrs)
           @report.logger.debug("from: #{query.from}, to: #{query.to}")
 
           create_inline(parent, :quoted, query.execute)
diff --git a/lib/grafana_reporter/asciidoctor/processor_mixin.rb b/lib/grafana_reporter/asciidoctor/processor_mixin.rb
index dc94733..b7cbb5c 100644
--- a/lib/grafana_reporter/asciidoctor/processor_mixin.rb
+++ b/lib/grafana_reporter/asciidoctor/processor_mixin.rb
@@ -12,12 +12,52 @@ def current_report(report)
         self
       end
 
-      # This method is called if a demo report shall be built for the given {Panel}.
-      # @param panel [Panel] panel object, for which a demo entry shall be created.
+      # This method is called if a demo report shall be built for the given {Grafana::Panel}.
+      # @param panel [Grafana::Panel] panel object, for which a demo entry shall be created.
       # @return [String] String containing the entry, or nil if not possible for given panel
       def build_demo_entry(panel)
         raise NotImplementedError
       end
+
+      # Sets default configurations from the given {Grafana::Dashboard} and store them as settings in the
+      # {AbstractQuery}.
+      #
+      # Following data is extracted:
+      # - +from+, by {Grafana::Dashboard#from_time}
+      # - +to+, by {Grafana::Dashboard#to_time}
+      # - and all variables as {Grafana::Variable}, prefixed with +var-+, as grafana also does it
+      # @param query [AbstractQuery] query object, for which the defaults are set
+      # @param dashboard [Grafana::Dashboard] dashboard from which the defaults are captured
+      def assign_dashboard_defaults(query, dashboard)
+        query.from = dashboard.from_time
+        query.to = dashboard.to_time
+        dashboard.variables.each { |item| query.assign_variable("var-#{item.name}", item) }
+      end
+
+      # Merges the given hashes to the given query object. It respects the priorities of the hashes and the
+      # object and allows only valid variables to be passed.
+      # @param query [AbstractQuery] query object, for which the defaults are set
+      # @param document_hash [Hash] variables from report template level
+      # @param item_hash [Hash] variables from item configuration level, i.e. specific call, which may override document
+      # @return [void]
+      def assign_doc_and_item_variables(query, document_hash, item_hash)
+        sel_doc_items = document_hash.select do |k, _v|
+          k =~ /^var-/ || k =~ /grafana_default_(?:from|to)_timezone/
+        end
+        sel_doc_items.each { |k, v| query.assign_variable(k, ::Grafana::Variable.new(v)) }
+        query.assign_variable('grafana_report_timestamp', ::Grafana::Variable.new(document_hash['localdatetime']))
+
+        sel_items = item_hash.select do |k, _v|
+          # TODO: specify accepted options in each class or check if simply all can be allowed with prefix +var-+
+          k =~ /^var-/ || k =~ /^render-/ || k =~ /filter_columns|format|replace_values_.*|transpose|column_divider|
+                                                   row_divider|from_timezone|to_timezone|result_type|query/x
+        end
+        sel_items.each { |k, v| query.assign_variable(k, ::Grafana::Variable.new(v)) }
+
+        query.timeout = item_hash['timeout'] || document_hash['grafana-default-timeout'] || query.timeout
+        query.from = item_hash['from'] || document_hash['from'] || query.from
+        query.to = item_hash['to'] || document_hash['to'] || query.to
+      end
     end
   end
 end
diff --git a/lib/grafana_reporter/asciidoctor/report.rb b/lib/grafana_reporter/asciidoctor/report.rb
index 386dde1..d58be28 100644
--- a/lib/grafana_reporter/asciidoctor/report.rb
+++ b/lib/grafana_reporter/asciidoctor/report.rb
@@ -6,22 +6,18 @@ module Asciidoctor
     # Implementation of a specific {AbstractReport}. It is used to
     # build reports specifically for asciidoctor results.
     class Report < ::GrafanaReporter::AbstractReport
-      # (see AbstractReport#initialize)
-      def initialize(config, template, destination_file_or_path = nil, custom_attributes = {})
+      # @see AbstractReport#initialize
+      def initialize(config)
         super
-        @current_pos = 0
         @image_files = []
-        @grafana_instances = {}
       end
 
-      # Starts to create an asciidoctor report. It utilizes all {Extensions} to
-      # realize the conversion.
+      # Starts to create an asciidoctor report. It utilizes all extensions in the {GrafanaReporter::Asciidoctor}
+      # namespace to realize the conversion.
       # @see AbstractReport#create_report
-      # @return [void]
-      def create_report
+      def create_report(template, destination_file_or_path = nil, custom_attributes = {})
         super
         attrs = { 'convert-backend' => 'pdf' }.merge(@config.default_document_attributes.merge(@custom_attributes))
-        attrs['grafana-report-timestamp'] = @start_time.to_s
         logger.debug("Document attributes: #{attrs}")
 
         initialize_step_counter
@@ -87,37 +83,17 @@ def create_report
         end
 
         clean_image_files
+      rescue MissingTemplateError => e
+        @logger.error(e.message)
+        @error = [e.message]
         done!
+        raise e
       rescue StandardError => e
         # catch all errors during execution
         died_with_error(e)
         raise e
-      end
-
-      # @see AbstractReport#progress
-      # @return [Float] number between 0 and 1 reflecting the current progress.
-      def progress
-        return 0 if @total_steps.to_i.zero?
-
-        @current_pos.to_f / @total_steps
-      end
-
-      # @param instance [String] requested grafana instance
-      # @return [Grafana::Grafana] the requested grafana instance.
-      def grafana(instance)
-        unless @grafana_instances[instance]
-          @grafana_instances[instance] = ::Grafana::Grafana.new(@config.grafana_host(instance),
-                                                                @config.grafana_api_key(instance),
-                                                                logger: @logger)
-        end
-        @grafana_instances[instance]
-      end
-
-      # Increments the progress.
-      # @return [Integer] number of the current progress position.
-      def next_step
-        @current_pos += 1
-        @current_pos
+      ensure
+        done!
       end
 
       # Called to save a temporary image file. After the final generation of the
@@ -126,6 +102,7 @@ def next_step
       # @return [String] path to the temporary file.
       def save_image_file(img_data)
         file = Tempfile.new(['gf_image_', '.png'], @config.images_folder.to_s)
+        file.binmode
         file.write(img_data)
         path = file.path.gsub(/#{@config.images_folder}/, '')
 
@@ -135,12 +112,11 @@ def save_image_file(img_data)
         path
       end
 
-      # Called, if the report generation has died with an error.
-      # @param error [StandardError] occured error
-      # @return [void]
-      def died_with_error(error)
-        @error = [error.message] << [error.backtrace]
-        done!
+      # @see AbstractReport#demo_report_classes
+      def self.demo_report_classes
+        [AlertsTableIncludeProcessor, AnnotationsTableIncludeProcessor, PanelImageBlockMacro, PanelImageInlineMacro,
+         PanelPropertyInlineMacro, PanelQueryTableIncludeProcessor, PanelQueryValueInlineMacro,
+         SqlTableIncludeProcessor, SqlValueInlineMacro, ShowHelpIncludeProcessor, ShowEnvironmentIncludeProcessor]
       end
 
       private
diff --git a/lib/grafana_reporter/asciidoctor/sql_table_include_processor.rb b/lib/grafana_reporter/asciidoctor/sql_table_include_processor.rb
index 642332b..571bba5 100644
--- a/lib/grafana_reporter/asciidoctor/sql_table_include_processor.rb
+++ b/lib/grafana_reporter/asciidoctor/sql_table_include_processor.rb
@@ -26,11 +26,11 @@ module Asciidoctor
     #
     # +to+ - 'to' time for the sql query
     #
-    # +format+ - see {QueryMixin#format_columns}
+    # +format+ - see {AbstractQuery#format_columns}
     #
-    # +replace_values+ - see {QueryMixin#replace_values}
+    # +replace_values+ - see {AbstractQuery#replace_values}
     #
-    # +filter_columns+ - see {QueryMixin#filter_columns}
+    # +filter_columns+ - see {AbstractQuery#filter_columns}
     class SqlTableIncludeProcessor < ::Asciidoctor::Extensions::IncludeProcessor
       include ProcessorMixin
 
@@ -54,7 +54,7 @@ def process(doc, reader, target, attrs)
           query = QueryValueQuery.new(@report.grafana(instance))
           query.datasource = @report.grafana(instance).datasource_by_id(target.split(':')[1].to_i)
           query.raw_query = attrs['sql']
-          query.merge_hash_variables(doc.attributes, attrs)
+          assign_doc_and_item_variables(query, doc.attributes, attrs)
           @report.logger.debug("from: #{query.from}, to: #{query.to}")
 
           reader.unshift_lines query.execute
diff --git a/lib/grafana_reporter/asciidoctor/sql_value_inline_macro.rb b/lib/grafana_reporter/asciidoctor/sql_value_inline_macro.rb
index 7e672d4..660051a 100644
--- a/lib/grafana_reporter/asciidoctor/sql_value_inline_macro.rb
+++ b/lib/grafana_reporter/asciidoctor/sql_value_inline_macro.rb
@@ -26,11 +26,11 @@ module Asciidoctor
     #
     # +to+ - 'to' time for the sql query
     #
-    # +format+ - see {QueryMixin#format_columns}
+    # +format+ - see {AbstractQuery#format_columns}
     #
-    # +replace_values+ - see {QueryMixin#replace_values}
+    # +replace_values+ - see {AbstractQuery#replace_values}
     #
-    # +filter_columns+ - see {QueryMixin#filter_columns}
+    # +filter_columns+ - see {AbstractQuery#filter_columns}
     class SqlValueInlineMacro < ::Asciidoctor::Extensions::InlineMacroProcessor
       include ProcessorMixin
       use_dsl
@@ -52,7 +52,7 @@ def process(parent, target, attrs)
           query = QueryValueQuery.new(@report.grafana(instance))
           query.datasource = @report.grafana(instance).datasource_by_id(target)
           query.raw_query = attrs['sql']
-          query.merge_hash_variables(parent.document.attributes, attrs)
+          assign_doc_and_item_variables(query, parent.document.attributes, attrs)
           @report.logger.debug("from: #{query.from}, to: #{query.to}")
 
           create_inline(parent, :quoted, query.execute)
diff --git a/lib/grafana_reporter/configuration.rb b/lib/grafana_reporter/configuration.rb
index c4c71ef..2925914 100644
--- a/lib/grafana_reporter/configuration.rb
+++ b/lib/grafana_reporter/configuration.rb
@@ -13,6 +13,10 @@ module GrafanaReporter
   class Configuration
     # @return [AbstractReport] specific report class, which should be used.
     attr_accessor :report_class
+    attr_accessor :logger
+
+    # Default file name for grafana reporter configuration file
+    DEFAULT_CONFIG_FILE_NAME = 'grafana_reporter.config'
 
     # Returned by {#mode} if only a connection test shall be executed.
     MODE_CONNECTION_TEST = 'test'
@@ -30,7 +34,15 @@ def initialize
       @logger = ::Logger.new($stderr, level: :info)
     end
 
-    attr_accessor :logger
+    # Reads a given configuration file.
+    # @param config_file [String] path to configuration file, defaults to DEFAULT_CONFIG_FILE_NAME
+    # @return [Hash] configuration hash to be set as {Configuration#config}
+    def load_config_from_file(config_file = nil)
+      config_file ||= DEFAULT_CONFIG_FILE_NAME
+      self.config = YAML.load_file(config_file)
+    rescue StandardError => e
+      raise ConfigurationError, "Could not read config file '#{config_file}' (Error: #{e.message})"
+    end
 
     # Used to overwrite the current configuration.
     def config=(new_config)
@@ -53,7 +65,7 @@ def mode
     def template
       return nil if get_config('default-document-attributes:var-template').nil?
 
-      "#{templates_folder}#{get_config('default-document-attributes:var-template')}.adoc"
+      "#{templates_folder}#{get_config('default-document-attributes:var-template')}"
     end
 
     # @return [String] destination filename for the report in {MODE_SINGLE_RENDER}.
diff --git a/lib/grafana_reporter/console_configuration_wizard.rb b/lib/grafana_reporter/console_configuration_wizard.rb
index cb4dee9..be93a90 100644
--- a/lib/grafana_reporter/console_configuration_wizard.rb
+++ b/lib/grafana_reporter/console_configuration_wizard.rb
@@ -31,7 +31,7 @@ def start_wizard(config_file, console_config)
       demo_report = create_demo_report(config)
 
       demo_report ||= '<<your_report_name>>'
-      config_param = config_file == Application::Application::CONFIG_FILE ? '' : " -c #{config_file}"
+      config_param = config_file == Configuration::DEFAULT_CONFIG_FILE_NAME ? '' : " -c #{config_file}"
       program_call = "#{Gem.ruby} #{$PROGRAM_NAME}"
       program_call = ENV['OCRA_EXECUTABLE'].gsub("#{Dir.pwd}/".gsub('/', '\\'), '') if ENV['OCRA_EXECUTABLE']
 
@@ -137,16 +137,8 @@ def create_demo_report(config)
         end
       end
 
-      # TODO: move this to Asciidoctor::Report class
-      classes = [Asciidoctor::AlertsTableIncludeProcessor, Asciidoctor::AnnotationsTableIncludeProcessor,
-                 Asciidoctor::PanelImageBlockMacro, Asciidoctor::PanelImageInlineMacro,
-                 Asciidoctor::PanelPropertyInlineMacro, Asciidoctor::PanelQueryTableIncludeProcessor,
-                 Asciidoctor::PanelQueryValueInlineMacro, Asciidoctor::SqlTableIncludeProcessor,
-                 Asciidoctor::SqlValueInlineMacro, Asciidoctor::ShowHelpIncludeProcessor,
-                 Asciidoctor::ShowEnvironmentIncludeProcessor]
-
       grafana = ::Grafana::Grafana.new(config.grafana_host, config.grafana_api_key)
-      demo_report_content = DemoReportWizard.new(classes).build(grafana)
+      demo_report_content = DemoReportWizard.new(config.report_class.demo_report_classes).build(grafana)
 
       begin
         File.write(demo_report_file, demo_report_content, mode: 'w')
diff --git a/lib/grafana_reporter/demo_report_wizard.rb b/lib/grafana_reporter/demo_report_wizard.rb
index f520120..b5ccd14 100644
--- a/lib/grafana_reporter/demo_report_wizard.rb
+++ b/lib/grafana_reporter/demo_report_wizard.rb
@@ -48,6 +48,8 @@ def evaluate_dashboard(dashboard, query_classes)
       results = {}
 
       dashboard.panels.shuffle.each do |panel|
+        next if panel.datasource.is_a?(Grafana::UnsupportedDatasource)
+
         query_classes.each do |query_class|
           unless query_class.public_instance_methods.include?(:build_demo_entry)
             results[query_class] = "Method 'build_demo_entry' not implemented for #{query_class.name}"
@@ -57,6 +59,11 @@ def evaluate_dashboard(dashboard, query_classes)
           begin
             result = query_class.new.build_demo_entry(panel)
             results[query_class] = result if result
+          rescue Grafana::DatasourceDoesNotExistError
+            # properly catch DatasourceDoesNotExist errors here, as they don't lead to a real issue
+            # during demo report creation
+            # This may e.g. happen if a panel asks e.g. for datasource '-- Dashboard --' which is
+            # currently not allowed
           rescue StandardError => e
             puts "#{e.message}\n#{e.backtrace.join("\n")}"
           end
diff --git a/lib/grafana_reporter/erb/report.rb b/lib/grafana_reporter/erb/report.rb
new file mode 100644
index 0000000..faf1d11
--- /dev/null
+++ b/lib/grafana_reporter/erb/report.rb
@@ -0,0 +1,43 @@
+# frozen_string_literal: true
+
+require 'erb'
+
+module GrafanaReporter
+  module ERB
+    # Implementation of a specific {AbstractReport}. It is used to
+    # build reports specifically for erb templates.
+    class Report < ::GrafanaReporter::AbstractReport
+      # Starts to create an asciidoctor report. It utilizes all extensions in the {GrafanaReporter::Asciidoctor}
+      # namespace to realize the conversion.
+      # @see AbstractReport#create_report
+      def create_report(template, destination_file_or_path = nil, custom_attributes = {})
+        super
+        attrs = @config.default_document_attributes.merge(@custom_attributes)
+        logger.debug("Document attributes: #{attrs}")
+
+        # TODO: if path is true, a default filename has to be generated. check if this should be a general function instead
+        @report = self
+        File.write(path, ::ERB.new(File.read(template)).result(binding))
+
+        # TODO: check if closing output file is correct here, or maybe can be moved to AbstractReport.done!
+        @destination_file_or_path.close if @destination_file_or_path.is_a?(File)
+      rescue MissingTemplateError => e
+        @logger.error(e.message)
+        @error = [e.message]
+        done!
+        raise e
+      rescue StandardError => e
+        # catch all errors during execution
+        died_with_error(e)
+        raise e
+      ensure
+        done!
+      end
+
+      # @see AbstractReport#demo_report_classes
+      def self.demo_report_classes
+        []
+      end
+    end
+  end
+end
diff --git a/lib/grafana_reporter/errors.rb b/lib/grafana_reporter/errors.rb
index 6b5ff32..ae0262f 100644
--- a/lib/grafana_reporter/errors.rb
+++ b/lib/grafana_reporter/errors.rb
@@ -8,6 +8,14 @@ def initialize(message)
     end
   end
 
+  # Raised if a datasource shall be queried, which is not (yet) supported by the reporter
+  class DatasourceNotSupportedError < GrafanaReporterError
+    def initialize(ds, query)
+      super("The datasource '#{ds.name}' is of type '#{ds.type}' which is currently not supported for "\
+            "the query type '#{query}'.")
+    end
+  end
+
   # Thrown, if the requested grafana instance does not have the mandatory 'host'
   # setting configured.
   class GrafanaInstanceWithoutHostError < GrafanaReporterError
@@ -46,7 +54,7 @@ def initialize(item, verb, expected, currently)
     end
   end
 
-  # Thrown, if the value configuration in {QueryMixin#replace_values} is
+  # Thrown, if the value configuration in {AbstractQuery#replace_values} is
   # invalid.
   class MalformedReplaceValuesStatementError < GrafanaReporterError
     def initialize(statement)
diff --git a/lib/grafana_reporter/help.rb b/lib/grafana_reporter/help.rb
index 9f9ed33..9e6aaa0 100644
--- a/lib/grafana_reporter/help.rb
+++ b/lib/grafana_reporter/help.rb
@@ -18,11 +18,6 @@ def github(headline_level = 2)
       "#{toc}\n\n#{help_text(github_options.merge(level: headline_level))}"
     end
 
-    # @see AbstractQuery#self.build_demo_entry
-    def self.build_demo_entry(_panel)
-      'include::grafana_help[]'
-    end
-
     private
 
     def github_options
diff --git a/lib/grafana_reporter/logger/two_way_logger.rb b/lib/grafana_reporter/logger/two_way_delegate_logger.rb
similarity index 100%
rename from lib/grafana_reporter/logger/two_way_logger.rb
rename to lib/grafana_reporter/logger/two_way_delegate_logger.rb
diff --git a/lib/grafana_reporter/panel_image_query.rb b/lib/grafana_reporter/panel_image_query.rb
index d2e8c87..5cc29a8 100644
--- a/lib/grafana_reporter/panel_image_query.rb
+++ b/lib/grafana_reporter/panel_image_query.rb
@@ -5,9 +5,9 @@ module GrafanaReporter
   class PanelImageQuery < AbstractQuery
     # Sets the proper render variables.
     def pre_process
-      @from = translate_date(@from, @variables['grafana-report-timestamp'], false, @variables['from_timezone'] ||
+      @from = translate_date(@from, @variables['grafana_report_timestamp'], false, @variables['from_timezone'] ||
                              @variables['grafana_default_from_timezone'])
-      @to = translate_date(@to, @variables['grafana-report-timestamp'], true, @variables['to_timezone'] ||
+      @to = translate_date(@to, @variables['grafana_report_timestamp'], true, @variables['to_timezone'] ||
                            @variables['grafana_default_to_timezone'])
       # TODO: ensure that in case of timezones are specified, that they are also forwarded to the image renderer
       # rename "render-" variables
diff --git a/lib/grafana_reporter/query_value_query.rb b/lib/grafana_reporter/query_value_query.rb
index 7e5050e..84d4ce9 100644
--- a/lib/grafana_reporter/query_value_query.rb
+++ b/lib/grafana_reporter/query_value_query.rb
@@ -8,15 +8,15 @@ class QueryValueQuery < AbstractQuery
     def pre_process
       @datasource = @panel.datasource if @panel
 
-      @from = translate_date(@from, @variables['grafana-report-timestamp'], false, @variables['from_timezone'] ||
+      @from = translate_date(@from, @variables['grafana_report_timestamp'], false, @variables['from_timezone'] ||
                              @variables['grafana_default_from_timezone'])
-      @to = translate_date(@to, @variables['grafana-report-timestamp'], true, @variables['to_timezone'] ||
+      @to = translate_date(@to, @variables['grafana_report_timestamp'], true, @variables['to_timezone'] ||
                            @variables['grafana_default_to_timezone'])
       @variables['result_type'] ||= Variable.new('')
     end
 
-    # Executes {QueryMixin#format_columns}, {QueryMixin#replace_values} and
-    # {QueryMixin#filter_columns} on the query results.
+    # Executes {AbstractQuery#format_columns}, {AbstractQuery#replace_values} and
+    # {AbstractQuery#filter_columns} on the query results.
     #
     # Finally the results are formatted as a asciidoctor table.
     # @see Grafana::AbstractQuery#post_process
diff --git a/lib/ruby_grafana_reporter.rb b/lib/ruby_grafana_reporter.rb
index 65c6468..2ec2ecc 100644
--- a/lib/ruby_grafana_reporter.rb
+++ b/lib/ruby_grafana_reporter.rb
@@ -24,6 +24,7 @@
   %w[grafana_reporter],
   %w[grafana_reporter asciidoctor extensions],
   %w[grafana_reporter asciidoctor],
+  %w[grafana_reporter erb],
   %w[grafana_reporter application]
 ]
 folders.each { |folder| Dir[File.join(__dir__, *folder, '*.rb')].sort.each { |file| require_relative file } }
diff --git a/ruby-grafana-reporter.gemspec b/ruby-grafana-reporter.gemspec
index 6b98915..5cd732f 100644
--- a/ruby-grafana-reporter.gemspec
+++ b/ruby-grafana-reporter.gemspec
@@ -7,6 +7,7 @@ folders = [
   %w[grafana_reporter],
   %w[grafana_reporter asciidoctor extensions],
   %w[grafana_reporter asciidoctor],
+  %w[grafana_reporter erb],
   %w[grafana_reporter application]
 ]
 
@@ -34,16 +35,16 @@ Gem::Specification.new do |s|
   }
 
   # the required ruby version is determined from the base docker image, currently debian stretch
-  s.required_ruby_version = '>=2.3.3'
+  s.required_ruby_version = '>=2.5'
   s.extra_rdoc_files = ['README.md', 'LICENSE']
 
   s.bindir = 'bin'
 
   s.add_runtime_dependency 'asciidoctor', '~>2.0'
-  s.add_runtime_dependency 'asciidoctor-pdf', '~>1.5'
+  s.add_runtime_dependency 'asciidoctor-pdf', '~>1.6'
   # the following package includes an interface to zip, which is also needed here
   # make sure that supported zip versions match - look in sub-dependency 'gepub'
-  #  s.add_runtime_dependency 'asciidoctor-epub3', '~>1.5.0'
+  #  s.add_runtime_dependency 'asciidoctor-epub3', '~>1.5.1'
   s.add_runtime_dependency 'rubyzip', '>1.1.1', '<2.4'
 
   s.add_development_dependency 'rspec', '~>3.9'
diff --git a/spec/integration/alerts_table_include_processor_spec.rb b/spec/integration/alerts_table_include_processor_spec.rb
index 22a0697..d250a73 100644
--- a/spec/integration/alerts_table_include_processor_spec.rb
+++ b/spec/integration/alerts_table_include_processor_spec.rb
@@ -5,7 +5,7 @@
     config = Configuration.new
     config.logger.level = ::Logger::Severity::WARN
     config.config = { 'grafana' => { 'default' => { 'host' => STUBS[:url], 'api_key' => STUBS[:key_admin] } } }
-    report = Report.new(config, './spec/tests/demo_report.adoc')
+    report = Report.new(config)
     Asciidoctor::Extensions.unregister_all
     Asciidoctor::Extensions.register do
       include_processor AlertsTableIncludeProcessor.new.current_report(report)
diff --git a/spec/integration/annotations_table_include_processor_spec.rb b/spec/integration/annotations_table_include_processor_spec.rb
index edc2467..9587af4 100644
--- a/spec/integration/annotations_table_include_processor_spec.rb
+++ b/spec/integration/annotations_table_include_processor_spec.rb
@@ -5,7 +5,7 @@
     config = Configuration.new
     config.logger.level = ::Logger::Severity::WARN
     config.config = { 'grafana' => { 'default' => { 'host' => STUBS[:url], 'api_key' => STUBS[:key_admin] } } }
-    report = Report.new(config, './spec/tests/demo_report.adoc')
+    report = Report.new(config)
     Asciidoctor::Extensions.unregister_all
     Asciidoctor::Extensions.register do
       include_processor AnnotationsTableIncludeProcessor.new.current_report(report)
diff --git a/spec/integration/application_spec.rb b/spec/integration/application_spec.rb
index a4742d9..cdd0038 100644
--- a/spec/integration/application_spec.rb
+++ b/spec/integration/application_spec.rb
@@ -81,6 +81,12 @@ def done?
       File.delete('./result.pdf') if File.exist?('./result.pdf')
     end
 
+    it 'can single render a template with extension' do
+      expect(subject.config.logger).not_to receive(:error)
+      expect { subject.configure_and_run(['-c', './spec/tests/demo_config.txt', '-t', 'spec/tests/demo_report.adoc', '-o', './result.pdf', '-d', 'ERROR']) }.not_to output(/ERROR/).to_stderr
+      expect(File.exist?('./result.pdf')).to be true
+    end
+
     it 'can single render a template and output to custom folder' do
       expect(subject.config.logger).not_to receive(:error)
       expect { subject.configure_and_run(['-c', './spec/tests/demo_config.txt', '-t', 'spec/tests/demo_report', '-o', './result.pdf', '-d', 'ERROR']) }.not_to output(/ERROR/).to_stderr
@@ -98,10 +104,57 @@ def done?
     end
 
     it 'does not raise error on non existing template' do
+      expect(subject.config.logger).to receive(:error).with(/is not a valid template/)
       expect { subject.configure_and_run(['-c', './spec/tests/demo_config.txt', '-t', 'does_not_exist']) }.to output(/report template .* is not a valid template/).to_stdout
     end
   end
 
+  context 'custom plugins' do
+    subject { GrafanaReporter::Application::Application.new }
+
+    before do
+      File.delete('./result.pdf') if File.exist?('./result.pdf')
+    end
+
+    after do
+      # remove temporary added plugin from respective places, so that other test cases run
+      # as if that would have never happened
+      expect(Object.constants.include?(:MyUnknownDatasource)).to be true
+      AbstractDatasource.class_eval('@@subclasses -= [MyUnknownDatasource]')
+      Object.send(:remove_const, :MyUnknownDatasource)
+      Object.send(:const_set, :MyUnknownDatasource, Class.new)
+    end
+
+    it 'can register and apply custom plugins' do
+      expect { subject.configure_and_run(['-c', './spec/tests/demo_config.txt', '-t', 'spec/tests/custom_demo_report', '-o', './result.pdf']) }.to output(/ERROR/).to_stderr
+      expect(subject.config.logger).not_to receive(:error)
+      expect { subject.configure_and_run(['-c', './spec/tests/demo_config.txt', '-t', 'spec/tests/custom_demo_report', '-o', './result.pdf', '-r', './spec/tests/custom_plugin']) }.not_to output(/ERROR/).to_stderr
+      expect(Object.constants.include?(:MyUnknownDatasource)).to be true
+    end
+  end
+
+  context 'ERB templating' do
+    subject { GrafanaReporter::Application::Application.new }
+
+    before do
+      File.delete('./result.txt') if File.exist?('./result.txt')
+      allow(subject.config.logger).to receive(:debug)
+      allow(subject.config.logger).to receive(:info)
+      allow(subject.config.logger).to receive(:warn)
+    end
+
+    after do
+      File.delete('./result.txt') if File.exist?('./result.txt')
+    end
+
+    it 'can single render a template with extension' do
+      expect(subject.config.logger).not_to receive(:error)
+      expect { subject.configure_and_run(['-c', './spec/tests/erb.config', '-t', 'spec/tests/erb.template', '-o', './result.txt', '-d', 'ERROR']) }.not_to output(/ERROR/).to_stderr
+      expect(File.exist?('./result.txt')).to be true
+      expect(File.read('./result.txt')).to include('This is a test 1594308060000.')
+    end
+  end
+
   context 'webserver' do
     before(:context) do
       WebMock.disable_net_connect!(allow: ['http://localhost:8033'])
@@ -136,8 +189,9 @@ def done?
     after(:context) do
       WebMock.enable!
       AbstractReport.clear_event_listeners
-      @webserver.kill
-      # TODO: kill webservice properly and release port again
+      # kill webservice properly and release port again
+      @app.webservice.stop!
+      sleep 0.1 until @app.webservice.stopped?
     end
 
     it 'responds to overview' do
@@ -242,14 +296,24 @@ def done?
       expect(res['content-disposition']).to include('.zip')
     end
 
-    it 'returns error on render without proper template' do
-      expect(@app.config.logger).to receive(:error).with(/is not a valid template\./)
+    it 'returns error on render without template' do
+      evt = ReportEventHandler.new
+      AbstractReport.add_event_listener(:on_after_finish, evt)
+
+      expect_any_instance_of(GrafanaReporter::Logger::TwoWayDelegateLogger).to receive(:error).with(/is not a valid template\./)
       res = Net::HTTP.get(URI('http://localhost:8033/render'))
-      expect(res).to include("is not a valid template.")
+      cur_time = Time.new
+      sleep 0.1 while !evt.done? && Time.new - cur_time < 10
+    end
 
-      expect(@app.config.logger).to receive(:error).with(/is not a valid template\./)
+    it 'returns error on render with non existing template' do
+      evt = ReportEventHandler.new
+      AbstractReport.add_event_listener(:on_after_finish, evt)
+
+      expect_any_instance_of(GrafanaReporter::Logger::TwoWayDelegateLogger).to receive(:error).with(/is not a valid template\./)
       res = Net::HTTP.get(URI('http://localhost:8033/render?var-template=does_not_exist'))
-      expect(res).to include("is not a valid template.")
+      cur_time = Time.new
+      sleep 0.1 while !evt.done? && Time.new - cur_time < 10
     end
   end
 end
diff --git a/spec/integration/panel_image_macro_spec.rb b/spec/integration/panel_image_macro_spec.rb
index 61eedd8..b8f2466 100644
--- a/spec/integration/panel_image_macro_spec.rb
+++ b/spec/integration/panel_image_macro_spec.rb
@@ -5,7 +5,7 @@
     config = Configuration.new
     config.logger.level = ::Logger::Severity::WARN
     config.config = { 'grafana' => { 'default' => { 'host' => STUBS[:url], 'api_key' => STUBS[:key_admin] } } }
-    report = Report.new(config, './spec/tests/demo_report.adoc')
+    report = Report.new(config)
     Asciidoctor::Extensions.unregister_all
     Asciidoctor::Extensions.register do
       block_macro PanelImageBlockMacro.new.current_report(report)
@@ -18,7 +18,13 @@
     expect(Asciidoctor.convert("grafana_panel_image::#{STUBS[:panel_sql][:id]}[dashboard=\"#{STUBS[:dashboard]}\"]", to_file: false)).to include('<img src="gf_image_').and match(/(?!Error)/)
   end
 
-  it 'shows errors properly' do
+  it 'can be processed for datasources with unknown type' do
+    expect(@report.logger).not_to receive(:error)
+    expect(@report.logger).not_to receive(:warn)
+    expect(Asciidoctor.convert("grafana_panel_image::#{STUBS[:panel_ds_unknown][:id]}[dashboard=\"#{STUBS[:dashboard]}\"]", to_file: false)).to include('<img src="gf_image_').and match(/(?!Error)/)
+  end
+
+  it 'shows errors properly if panel is unknown' do
     expect(@report.logger).to receive(:fatal)
     expect(Asciidoctor.convert("grafana_panel_image::999[dashboard=\"#{STUBS[:dashboard]}\"]", to_file: false)).to include('Error')
   end
@@ -29,7 +35,7 @@
     config = Configuration.new
     config.config = { 'grafana' => { 'default' => { 'host' => STUBS[:url], 'api_key' => STUBS[:key_admin] } } }
     config.logger.level = ::Logger::Severity::WARN
-    report = Report.new(config, './spec/tests/demo_report.adoc')
+    report = Report.new(config)
     Asciidoctor::Extensions.unregister_all
     Asciidoctor::Extensions.register do
       inline_macro PanelImageInlineMacro.new.current_report(report)
@@ -44,8 +50,7 @@
 
   it 'cleans up created temporary files' do
     expect(@report.logger).not_to receive(:error)
-    ts = Time.now.to_s
-    result = Asciidoctor.convert("grafana_panel_image:#{STUBS[:panel_sql][:id]}[dashboard=\"#{STUBS[:dashboard]}\"]", to_file: false, attributes: { 'grafana-report-timestamp' => ts })
+    result = Asciidoctor.convert("grafana_panel_image:#{STUBS[:panel_sql][:id]}[dashboard=\"#{STUBS[:dashboard]}\"]", to_file: false)
     tmp_file = result.to_s.gsub(/.*img src="([^"]+)".*/m, '\1')
 # TODO: ensure that the file existed before
     expect(File.exist?("./spec/templates/images/#{tmp_file}")).to be false
diff --git a/spec/integration/panel_property_inline_macro_spec.rb b/spec/integration/panel_property_inline_macro_spec.rb
index 7e887b5..73ba48f 100644
--- a/spec/integration/panel_property_inline_macro_spec.rb
+++ b/spec/integration/panel_property_inline_macro_spec.rb
@@ -5,7 +5,7 @@
     config = Configuration.new
     config.logger.level = ::Logger::Severity::WARN
     config.config = { 'grafana' => { 'default' => { 'host' => STUBS[:url], 'api_key' => STUBS[:key_admin]} } }
-    report = Report.new(config, './spec/tests/demo_report.adoc')
+    report = Report.new(config)
     Asciidoctor::Extensions.unregister_all
     Asciidoctor::Extensions.register do
       inline_macro PanelPropertyInlineMacro.new.current_report(report)
diff --git a/spec/integration/panel_query_table_include_processor_spec.rb b/spec/integration/panel_query_table_include_processor_spec.rb
index abc29ea..34527b2 100644
--- a/spec/integration/panel_query_table_include_processor_spec.rb
+++ b/spec/integration/panel_query_table_include_processor_spec.rb
@@ -5,7 +5,7 @@
     config = Configuration.new
     config.logger.level = ::Logger::Severity::WARN
     config.config = { 'grafana' => { 'default' => { 'host' => STUBS[:url], 'api_key' => STUBS[:key_admin]} } }
-    report = Report.new(config, './spec/tests/demo_report.adoc')
+    report = Report.new(config)
     Asciidoctor::Extensions.unregister_all
     Asciidoctor::Extensions.register do
       include_processor PanelQueryTableIncludeProcessor.new.current_report(report)
@@ -73,6 +73,13 @@
 
   end
 
+  context 'unknown datasource' do
+    it 'returns error on unknown datasource requests' do
+      expect(@report.logger).to receive(:error)
+      expect(Asciidoctor.convert("include::grafana_panel_query_table:#{STUBS[:panel_ds_unknown][:id]}[query=\"A\",dashboard=\"#{STUBS[:dashboard]}\",from=\"0\",to=\"0\"]", to_file: false)).to include('Error')
+    end
+  end
+
   context 'graphite' do
     it 'can handle graphite requests' do
       expect(@report.logger).not_to receive(:error)
diff --git a/spec/integration/panel_query_value_inline_macro_spec.rb b/spec/integration/panel_query_value_inline_macro_spec.rb
index 3aa495f..fe252e0 100644
--- a/spec/integration/panel_query_value_inline_macro_spec.rb
+++ b/spec/integration/panel_query_value_inline_macro_spec.rb
@@ -5,7 +5,7 @@
     config = Configuration.new
     config.logger.level = ::Logger::Severity::WARN
     config.config = { 'grafana' => { 'default' => { 'host' => STUBS[:url], 'api_key' => STUBS[:key_admin] } } }
-    report = Report.new(config, './spec/tests/demo_report.adoc')
+    report = Report.new(config)
     Asciidoctor::Extensions.unregister_all
     Asciidoctor::Extensions.register do
       inline_macro PanelQueryValueInlineMacro.new.current_report(report)
@@ -60,6 +60,11 @@
     expect(Asciidoctor.convert("grafana_panel_query_value:#{STUBS[:panel_sql][:id]}[query=\"#{STUBS[:panel_sql][:letter]}\",dashboard=\"#{STUBS[:dashboard]}\",format=\",%.2f\",filter_columns=\"time_sec\"]", to_file: false)).to include('<p>43.90')
   end
 
+  it 'can filter columns and handle wront format definitions' do
+    expect(@report.logger).to receive(:error).with('invalid format character - %').at_least(:once)
+    expect(Asciidoctor.convert("grafana_panel_query_value:#{STUBS[:panel_sql][:id]}[query=\"#{STUBS[:panel_sql][:letter]}\",dashboard=\"#{STUBS[:dashboard]}\",format=\",%2%2f\",filter_columns=\"time_sec\"]", to_file: false)).to include('<p>invalid format character')
+  end
+
   it 'shows fatal error if query is missing' do
     expect(@report.logger).to receive(:fatal).with(/GrafanaError: The specified query '' does not exist in the panel '11' in dashboard.*/)
     expect(Asciidoctor.convert("grafana_panel_query_value:#{STUBS[:panel_sql][:id]}[dashboard=\"#{STUBS[:dashboard]}\",format=\",%.2f\",filter_columns=\"time_sec\"]", to_file: false)).to include('GrafanaError: The specified query \'\' does not exist in the panel \'11\' in dashboard')
diff --git a/spec/integration/report_webhook_spec.rb b/spec/integration/report_webhook_spec.rb
index fe419f1..21c9d35 100644
--- a/spec/integration/report_webhook_spec.rb
+++ b/spec/integration/report_webhook_spec.rb
@@ -32,8 +32,9 @@
     after(:context) do
       WebMock.enable!
       AbstractReport.clear_event_listeners
-      @webserver.kill
-      # TODO: kill webservice properly and release port again
+      # kill webservice properly and release port again
+      @app.webservice.stop!
+      sleep 0.1 until @app.webservice.stopped?
     end
 
     it 'calls event listener properly' do
diff --git a/spec/integration/show_environment_include_processor_spec.rb b/spec/integration/show_environment_include_processor_spec.rb
index ecb6c74..1fb5e38 100644
--- a/spec/integration/show_environment_include_processor_spec.rb
+++ b/spec/integration/show_environment_include_processor_spec.rb
@@ -5,7 +5,7 @@
     config = Configuration.new
     config.logger.level = ::Logger::Severity::WARN
     config.config = { 'grafana' => { 'default' => { 'host' => STUBS[:url], 'api_key' => STUBS[:key_admin] } } }
-    report = Report.new(config, './spec/tests/demo_report.adoc')
+    report = Report.new(config)
     Asciidoctor::Extensions.unregister_all
     Asciidoctor::Extensions.register do
       include_processor ShowEnvironmentIncludeProcessor.new.current_report(report)
diff --git a/spec/integration/show_help_include_processor_spec.rb b/spec/integration/show_help_include_processor_spec.rb
index 7f49019..3757098 100644
--- a/spec/integration/show_help_include_processor_spec.rb
+++ b/spec/integration/show_help_include_processor_spec.rb
@@ -5,7 +5,7 @@
     config = Configuration.new
     config.logger.level = ::Logger::Severity::WARN
     config.config = { 'grafana' => { 'default' => { 'host' => STUBS[:url], 'api_key' => STUBS[:key_admin] } } }
-    report = Report.new(config, './spec/tests/demo_report.adoc')
+    report = Report.new(config)
     Asciidoctor::Extensions.unregister_all
     Asciidoctor::Extensions.register do
       include_processor ShowHelpIncludeProcessor.new.current_report(report)
diff --git a/spec/integration/sql_table_include_processor_spec.rb b/spec/integration/sql_table_include_processor_spec.rb
index 97ad612..a818949 100644
--- a/spec/integration/sql_table_include_processor_spec.rb
+++ b/spec/integration/sql_table_include_processor_spec.rb
@@ -5,7 +5,7 @@
     config = Configuration.new
     config.logger.level = ::Logger::Severity::WARN
     config.config = { 'grafana' => { 'default' => { 'host' => STUBS[:url], 'api_key' => STUBS[:key_admin] } } }
-    report = Report.new(config, './spec/tests/demo_report.adoc')
+    report = Report.new(config)
     Asciidoctor::Extensions.unregister_all
     Asciidoctor::Extensions.register do
       include_processor SqlTableIncludeProcessor.new.current_report(report)
diff --git a/spec/integration/sql_value_inline_macro_spec.rb b/spec/integration/sql_value_inline_macro_spec.rb
index 79478fe..052de6b 100644
--- a/spec/integration/sql_value_inline_macro_spec.rb
+++ b/spec/integration/sql_value_inline_macro_spec.rb
@@ -5,7 +5,7 @@
     config = Configuration.new
     config.logger.level = ::Logger::Severity::WARN
     config.config = { 'grafana' => { 'default' => { 'host' => STUBS[:url], 'api_key' => STUBS[:key_admin] } } }
-    report = Report.new(config, './spec/tests/demo_report.adoc')
+    report = Report.new(config)
     Asciidoctor::Extensions.unregister_all
     Asciidoctor::Extensions.register do
       inline_macro SqlValueInlineMacro.new.current_report(report)
diff --git a/spec/integration/value_as_variable_include_processor_spec.rb b/spec/integration/value_as_variable_include_processor_spec.rb
index 6f0e3ec..77d6638 100644
--- a/spec/integration/value_as_variable_include_processor_spec.rb
+++ b/spec/integration/value_as_variable_include_processor_spec.rb
@@ -5,7 +5,7 @@
     config = Configuration.new
     config.logger.level = ::Logger::Severity::WARN
     config.config = { 'grafana' => { 'default' => { 'host' => STUBS[:url], 'api_key' => STUBS[:key_admin] } } }
-    report = Report.new(config, './spec/tests/demo_report.adoc')
+    report = Report.new(config)
     Asciidoctor::Extensions.unregister_all
     Asciidoctor::Extensions.register do
       include_processor ValueAsVariableIncludeProcessor.new.current_report(report)
diff --git a/spec/models/abstract_datasource_spec.rb b/spec/models/abstract_datasource_spec.rb
index 03ee688..5f02c0d 100644
--- a/spec/models/abstract_datasource_spec.rb
+++ b/spec/models/abstract_datasource_spec.rb
@@ -12,7 +12,7 @@
     expect { AbstractDatasource.build_instance(nil) }.to raise_error(InvalidDatasourceQueryProvidedError)
   end
 
-  it 'raises error if unknown datasource definition is provided' do
-    expect { AbstractDatasource.build_instance({'meta' => {'category' => 'unknown', 'id' => 'unknown_ds'}}) }.to raise_error(DatasourceTypeNotSupportedError)
+  it 'returns unsupported datasource if not supported' do
+    expect(AbstractDatasource.build_instance({'meta' => {'category' => 'unknown', 'id' => 'unknown_ds'}})).to be_a(Grafana::UnsupportedDatasource)
   end
 end
diff --git a/spec/models/abstract_report_spec.rb b/spec/models/abstract_report_spec.rb
index 6da4f73..dd73ddc 100644
--- a/spec/models/abstract_report_spec.rb
+++ b/spec/models/abstract_report_spec.rb
@@ -1,9 +1,9 @@
 include GrafanaReporter
 
 describe AbstractReport do
-  subject { AbstractReport.new(Configuration.new, './spec/tests/demo_report.adoc') }
+  subject { AbstractReport.new(Configuration.new) }
 
   it 'has abstract methods' do
-    expect { subject.progress }.to raise_error(NotImplementedError)
+    expect { subject.class.demo_report_classes }.to raise_error(NotImplementedError)
   end
 end
diff --git a/spec/models/dashboard_spec.rb b/spec/models/dashboard_spec.rb
index 9493abd..b203b1b 100644
--- a/spec/models/dashboard_spec.rb
+++ b/spec/models/dashboard_spec.rb
@@ -4,7 +4,7 @@
   let(:dashboard) { Dashboard.new(JSON.parse(File.read('./spec/tests/demo_dashboard.json'))['dashboard'], Grafana::Grafana.new('')) }
 
   it 'contains panels' do
-    expect(dashboard.panels.length).to eq(9)
+    expect(dashboard.panels.length).to eq(10)
     expect(dashboard.panel(11)).to be_a(Panel)
     expect(dashboard.panel(11).field('id')).to eq(11)
     expect(dashboard.panel(11).field('no_field_exists')).to eq('')
diff --git a/spec/stubs/webmock.rb b/spec/stubs/webmock.rb
index 0e8b49b..5bfa8c6 100644
--- a/spec/stubs/webmock.rb
+++ b/spec/stubs/webmock.rb
@@ -8,6 +8,7 @@
   key_admin: 'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx',
   key_viewer: 'viewerxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx',
   dashboard: 'IDBRfjSmz',
+  panel_ds_unknown: { id: '10' },
   panel_sql: { id: '11', letter: 'A', title: 'Temperaturen' },
   panel_graphite: { id: '12', letter: 'A' },
   panel_prometheus: { id: '13', letter: 'A' },
@@ -110,13 +111,13 @@
     )
     .to_return(status: 200, body: '{"results":{"A":{"refId":"A","meta":{"rowCount":0,"sql":"SELECT 1 as value WHERE value = 0"},"series":null,"tables":null,"dataframes":null}}}', headers: {})
 
-    stub_request(:get, %r{http://localhost/render/d-solo/IDBRfjSmz\?from=\d+&fullscreen=true&panelId=11&theme=light&timeout=60(?:&var-[^&]+)*}).with(
+    stub_request(:get, %r{http://localhost/render/d-solo/IDBRfjSmz\?from=\d+&fullscreen=true&panelId=(?:10|11)&theme=light&timeout=60(?:&var-[^&]+)*}).with(
       headers: default_header.merge({
         'Accept' => 'image/png',
         'Authorization' => "Bearer #{STUBS[:key_admin]}"
       })
     )
-    .to_return(status: 200, body: File.read('./spec/tests/sample_image.png'), headers: {})
+    .to_return(status: 200, body: File.read('./spec/tests/sample_image.png', File.size('./spec/tests/sample_image.png')), headers: {})
 
     stub_request(:post, 'http://localhost/api/tsdb/query').with(
       body: %r{.*SELECT   time as time_sec,   value / 10 as Ist FROM istwert_hk1 WHERE \$__unixEpochFilter\(time\) ORDER BY time DESC.*},
diff --git a/spec/tests/custom_demo_report.adoc b/spec/tests/custom_demo_report.adoc
new file mode 100644
index 0000000..f77f568
--- /dev/null
+++ b/spec/tests/custom_demo_report.adoc
@@ -0,0 +1,3 @@
+= Ruby-Grafana-Reporter Demo Report
+
+Show a valid value: grafana_sql_value:5[sql="SELECT 1"]
diff --git a/spec/tests/custom_plugin.rb b/spec/tests/custom_plugin.rb
new file mode 100644
index 0000000..fd60e4e
--- /dev/null
+++ b/spec/tests/custom_plugin.rb
@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class MyUnknownDatasource < ::Grafana::AbstractDatasource
+  def self.handles?(model)
+    model['type'] == 'UnknownDatasource'
+  end
+
+  def request(query_description)
+    { header: ['I am handled'], content: [[1000]] }
+  end
+end
diff --git a/spec/tests/demo_dashboard.json b/spec/tests/demo_dashboard.json
index aaf5bd0..65f6fe1 100644
--- a/spec/tests/demo_dashboard.json
+++ b/spec/tests/demo_dashboard.json
@@ -46,6 +46,18 @@
     "id": 2,
     "links": [],
     "panels": [
+      {
+        "datasource": "UnknownDatasource",
+        "id": 10,
+        "targets": [
+          {
+            "hide": false,
+            "refId": "A",
+            "target": "alias(movingAverage(scaleToSeconds(apps.fakesite.web_server_01.counters.request_status.code_302.count, 10), 20), 'cpu')"
+          }
+        ],
+        "type": "singlestat"
+      },
       {
         "datasource": "Graphite",
         "id": 12,
diff --git a/spec/tests/demo_dashboard_with_unknown_datasource.json b/spec/tests/demo_dashboard_with_unknown_datasource.json
deleted file mode 100644
index c762d27..0000000
--- a/spec/tests/demo_dashboard_with_unknown_datasource.json
+++ /dev/null
@@ -1,1175 +0,0 @@
-{
-  "meta": {
-    "isHome": true,
-    "canSave": false,
-    "canEdit": true,
-    "canAdmin": false,
-    "canStar": false,
-    "slug": "",
-    "url": "",
-    "expires": "0001-01-01T00:00:00Z",
-    "created": "0001-01-01T00:00:00Z",
-    "updated": "0001-01-01T00:00:00Z",
-    "updatedBy": "",
-    "createdBy": "",
-    "version": 0,
-    "hasAcl": false,
-    "isFolder": false,
-    "folderId": 0,
-    "folderTitle": "General",
-    "folderUrl": "",
-    "provisioned": false,
-    "provisionedExternalId": ""
-  },
-  "dashboard": {
-    "annotations": {
-      "list": [
-        {
-          "builtIn": 1,
-          "datasource": "-- Grafana --",
-          "enable": true,
-          "hide": false,
-          "iconColor": "rgba(0, 211, 255, 1)",
-          "limit": 100,
-          "name": "Annotations & Alerts",
-          "showIn": 0,
-          "tags": [
-            "all"
-          ],
-          "type": "tags"
-        }
-      ]
-    },
-    "editable": true,
-    "gnetId": null,
-    "graphTooltip": 0,
-    "id": 2,
-    "links": [],
-    "panels": [
-      {
-        "datasource": "DatasourceDoesNotExist",
-        "id": 12,
-        "targets": [
-          {
-            "hide": false,
-            "refId": "A",
-            "target": "alias(movingAverage(scaleToSeconds(apps.fakesite.web_server_01.counters.request_status.code_302.count, 10), 20), 'cpu')"
-          }
-        ],
-        "type": "singlestat"
-      },
-      {
-        "datasource": "Prometheus",
-        "id": 13,
-        "targets": [
-          {
-            "hide": false,
-            "expr": "sum by(mode)(irate(node_cpu_seconds_total{job=\"node\", instance=~\"$node:.*\", mode!=\"idle\"}[5m])) > 0",
-            "format": "time_series",
-            "interval": "",
-            "intervalFactor": 2,
-            "legendFormat": "{{mode}}",
-            "metric": "",
-            "refId": "A",
-            "step": 10
-          }
-        ],
-        "title": "CPU usage",
-        "type": "graph"
-      },
-      {
-        "datasource": "demo",
-        "aliasColors": {},
-        "bars": false,
-        "dashLength": 10,
-        "dashes": false,
-        "fill": 1,
-        "gridPos": {
-          "h": 9,
-          "w": 12,
-          "x": 0,
-          "y": 8
-        },
-        "id": 21,
-        "legend": {
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-            "selected": false,
-            "text": "ten",
-            "value": "10"
-          },
-          "hide": 0,
-          "includeAll": false,
-          "label": null,
-          "multi": false,
-          "name": "test",
-          "options": [
-            {
-              "selected": false,
-              "text": "one",
-              "value": "1"
-            },
-            {
-              "selected": true,
-              "text": "ten",
-              "value": "10"
-            },
-            {
-              "selected": false,
-              "text": "twenty-five",
-              "value": "25"
-            }
-          ],
-          "query": "1,10,25",
-          "skipUrlSync": false,
-          "type": "custom"
-        },
-        {
-          "allValue": null,
-          "current": {
-            "selected": true,
-            "text": "1 + 2 + , + $ + / + \" + ' + . + a + | + \\",
-            "value": [
-              "1",
-              "2",
-              ",",
-              "$",
-              "/",
-              "\"",
-              "'",
-              ".",
-              "a",
-              "|",
-              "\\"
-            ]
-          },
-          "hide": 0,
-          "includeAll": false,
-          "label": null,
-          "multi": true,
-          "name": "testmulti",
-          "options": [
-            {
-              "selected": true,
-              "text": "1",
-              "value": "1"
-            },
-            {
-              "selected": true,
-              "text": "2",
-              "value": "2"
-            },
-            {
-              "selected": true,
-              "text": ",",
-              "value": ","
-            },
-            {
-              "selected": true,
-              "text": "$",
-              "value": "$"
-            },
-            {
-              "selected": true,
-              "text": "/",
-              "value": "/"
-            },
-            {
-              "selected": true,
-              "text": "\"",
-              "value": "\""
-            },
-            {
-              "selected": true,
-              "text": "'",
-              "value": "'"
-            },
-            {
-              "selected": true,
-              "text": ".",
-              "value": "."
-            },
-            {
-              "selected": true,
-              "text": "a",
-              "value": "a"
-            },
-            {
-              "selected": true,
-              "text": "|",
-              "value": "|"
-            },
-            {
-              "selected": true,
-              "text": "\\",
-              "value": "\\"
-            }
-          ],
-          "query": "1,2,\\,,$,/,\",',.,a,|,\\",
-          "queryValue": "",
-          "skipUrlSync": false,
-          "type": "custom"
-        },
-        {
-          "allValue": null,
-          "current": {
-            "selected": true,
-            "text": "test bla12$/\"'.a,|=\\",
-            "value": "test bla12$/\"'.a,|=\\"
-          },
-          "hide": 0,
-          "includeAll": false,
-          "label": null,
-          "multi": false,
-          "name": "testsingle",
-          "options": [
-            {
-              "selected": true,
-              "text": "test bla12$/\"'.a,|=\\",
-              "value": "test bla12$/\"'.a,|=\\"
-            },
-            {
-              "selected": false,
-              "text": "resolved",
-              "value": "1"
-            }
-          ],
-          "query": "test bla12$/\"'.a\\,|=\\",
-          "queryValue": "",
-          "skipUrlSync": false,
-          "type": "custom"
-        },
-        {
-          "allValue": null,
-          "current": {
-            "selected": true,
-            "text": "1596660163000",
-            "value": "1596660163000"
-          },
-          "hide": 0,
-          "includeAll": false,
-          "label": null,
-          "multi": false,
-          "name": "timestamp",
-          "options": [
-            {
-              "selected": true,
-              "text": "1596660163000",
-              "value": "1596660163000"
-            }
-          ],
-          "query": "1596660163000",
-          "queryValue": "",
-          "skipUrlSync": false,
-          "type": "custom"
-        }
-      ]
-    },
-    "time": {
-      "from": "now-24h",
-      "to": "now"
-    },
-    "timepicker": {
-      "refresh_intervals": [
-        "5s",
-        "10s",
-        "30s",
-        "1m",
-        "5m",
-        "15m",
-        "30m",
-        "1h",
-        "2h",
-        "1d"
-      ],
-      "time_options": [
-        "5m",
-        "15m",
-        "1h",
-        "6h",
-        "12h",
-        "24h",
-        "2d",
-        "7d",
-        "30d"
-      ]
-    },
-    "timezone": "",
-    "title": "Todayd",
-    "uid": "IDBRfjSmz",
-    "version": 30
-  }
-}
-
diff --git a/spec/tests/erb.config b/spec/tests/erb.config
new file mode 100644
index 0000000..70d6930
--- /dev/null
+++ b/spec/tests/erb.config
@@ -0,0 +1,19 @@
+# This configuration has been built with the configuration wizard.
+
+grafana:
+  default:
+    host: http://localhost
+    api_key: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
+
+
+
+grafana-reporter:
+  report-class: GrafanaReporter::ERB::Report
+  templates-folder: .
+  reports-folder: .
+  report-retention: 24
+  webservice-port: 8015
+
+default-document-attributes:
+  imagesdir: .
+# feel free to add here additional asciidoctor document attributes which are applied to all your templates
diff --git a/spec/tests/erb.template b/spec/tests/erb.template
new file mode 100644
index 0000000..fc2276e
--- /dev/null
+++ b/spec/tests/erb.template
@@ -0,0 +1,9 @@
+<%
+dashboard = 'IDBRfjSmz'
+instance = 'default'
+panel = @report.grafana(instance).dashboard(dashboard).panel(11)
+query = QueryValueQuery.new(panel)
+query.assign_variable('result_type', ::Grafana::Variable.new('panel_value'))
+query.assign_variable('query', ::Grafana::Variable.new('A'))
+%>
+This is a test <%= query.execute %>.
diff --git a/spec/tests/frontend_settings.json b/spec/tests/frontend_settings.json
index d00a6cc..356b465 100644
--- a/spec/tests/frontend_settings.json
+++ b/spec/tests/frontend_settings.json
@@ -231,6 +231,18 @@
       "name": "Prometheus",
       "type": "prometheus",
       "url":"/api/datasources/proxy/4"
+    },
+    "UnknownDatasource": {
+      "id": 5,
+      "meta": {
+        "type": "datasource",
+        "name": "UnknownDatasource",
+        "id": "UnknownDatasource",
+        "category":"tsdb"
+      },
+      "name": "UnknownDatasource",
+      "type": "UnknownDatasource",
+      "url":"/api/datasources/proxy/4"
     }
   },
   "defaultDatasource": "demo",