using changedTouches instead of touches

In testing on different devices it seems changedTouches is better supported cross platform, using this instead of the the touches object. 
Roughly tested on iOS and Android.

src/dygraph-utils.js

@@ -193,7 +193,7 @@ Dygraph.findPos = function(obj) {
  * @private
  */
 Dygraph.pageX = function(e) {
-  if (e.isTouchOver) return (!e.touches[0] || e.touches[0].pageX < 0) ? 0 : e.touches[0].pageX;
+  if (e.isTouchOver) return (!e.changedTouches[0] || e.changedTouches[0].pageX < 0) ? 0 : e.changedTouches[0].pageX;
   return (!e.pageX || e.pageX < 0) ? 0 : e.pageX;
 };
 
@@ -206,7 +206,7 @@ Dygraph.pageX = function(e) {
  * @private
  */
 Dygraph.pageY = function(e) {
-  if (e.isTouchOver) return (!e.touches[0] || e.touches[0].pageY < 0) ? 0 : e.touches[0].pageY;
+  if (e.isTouchOver) return (!e.changedTouches[0] || e.changedTouches[0].pageY < 0) ? 0 : e.changedTouches[0].pageY;
   return (!e.pageY || e.pageY < 0) ? 0 : e.pageY;
 };