In startTouch(), pageX, pageY is not appropriate for the calculation …

…of dataX, dataY. This causes serious problems in the calculation of context.initialPinchCenter. The wrong calculations affect pinch-zoom when the x or y range is small and the graph div is farther to the bottom or right of the page.

For example, see http://jsfiddle.net/j63zvups/1/ and try to pinch-zoom in vertical, for scaling the y axis. You will get an ungovernable behaviour.

The solution is to use t.clientX and t.target.getBoundingClientRect().

To see that the old calculations were incorrect, consider this:

toDataYCoord(y) = yRange[0] + (area.y + area.h - y) / area.h * (yRange[1] - yRange[0]);

Now, if you pass y = pageY, you can see that (area.y + area.h - pageY) / area.h is wrong when pageY > area.y + area.h, which is the case when you scroll far enough.

src/dygraph-interaction-model.js

@@ -418,12 +418,13 @@ DygraphInteraction.startTouch = function(event, g, context) {
   var touches = [];
   for (var i = 0; i < event.touches.length; i++) {
     var t = event.touches[i];
+    var rect = t.target.getBoundingClientRect()
     // we dispense with 'dragGetX_' because all touchBrowsers support pageX
     touches.push({
       pageX: t.pageX,
       pageY: t.pageY,
-      dataX: g.toDataXCoord(t.pageX),
-      dataY: g.toDataYCoord(t.pageY)
+      dataX: g.toDataXCoord(t.clientX-rect.left),
+      dataY: g.toDataYCoord(t.clientY-rect.top)
       // identifier: t.identifier
     });
   }