code: function(event, g, context) {
      // Right-click should not initiate a zoom.
      if (event.button && event.button == 2) return;
  
      context.initializeMouseDown(event, g, context);
  
      if (event.altKey || event.shiftKey) {
        DygraphInteraction.startPan(event, g, context);
      } else {
        DygraphInteraction.startZoom(event, g, context);
      }
  
      // Note: we register mousemove/mouseup on document to allow some leeway for
      // events to move outside of the chart. Interaction model events get
      // registered on the canvas, which is too small to allow this.
      var mousemove = function(event) {
        if (context.isZooming) {
          // When the mouse moves >200px from the chart edge, cancel the zoom.
          var d = distanceFromChart(event, g);
          if (d < DRAG_EDGE_MARGIN) {
            DygraphInteraction.moveZoom(event, g, context);
          } else {
            if (context.dragEndX !== null) {
              context.dragEndX = null;
              context.dragEndY = null;
              g.clearZoomRect_();
            }
          }
        } else if (context.isPanning) {
          DygraphInteraction.movePan(event, g, context);
        }
      };
      var mouseup = function(event) {
        if (context.isZooming) {
          if (context.dragEndX !== null) {
            DygraphInteraction.endZoom(event, g, context);
          } else {
            DygraphInteraction.maybeTreatMouseOpAsClick(event, g, context);
          }
        } else if (context.isPanning) {
          DygraphInteraction.endPan(event, g, context);
        }
  
        utils.removeEvent(document, 'mousemove', mousemove);
        utils.removeEvent(document, 'mouseup', mouseup);
        context.destroy();
      };
  
      g.addAndTrackEvent(document, 'mousemove', mousemove);
      g.addAndTrackEvent(document, 'mouseup', mouseup);
    }
docs: Track the beginning of drag events