src/dygraph-interaction-model.js
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);
      }
    }
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);
      }
    }
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.