Don't trigger zoom outside the area of chart lines/bars

If user starts mouse down for example in area of legends or axis the zoom starts. This fix limits the starting of zooming to the area of the actual graph.

Fixes chartjs#256

src/plugin.js

@@ -447,8 +447,13 @@ var zoomPlugin = {
 		var panThreshold = options.pan && options.pan.threshold;
 
 		chartInstance.$zoom._mouseDownHandler = function(event) {
-			node.addEventListener('mousemove', chartInstance.$zoom._mouseMoveHandler);
-			chartInstance.$zoom._dragZoomStart = event;
+			var rect = event.target.getBoundingClientRect();
+			var offsetX = event.clientX - rect.left;
+			var offsetY = event.clientY - rect.top;
+			if (helpers.canvas._isPointInArea({x: offsetX, y: offsetY }, chartInstance.chartArea)) {
+				node.addEventListener('mousemove', chartInstance.$zoom._mouseMoveHandler);
+				chartInstance.$zoom._dragZoomStart = event;
+			}
 		};
 
 		chartInstance.$zoom._mouseMoveHandler = function(event) {