src/dygraph.js
function(event, g, contextB) {
      // prevents mouse drags from selecting page text.
      if (event.preventDefault) {
        event.preventDefault();  // Firefox, Chrome, etc.
      } else {
        event.returnValue = false;  // IE
        event.cancelBubble = true;
      }

      var canvasPos = utils.findPos(g.canvas_);
      contextB.px = canvasPos.x;
      contextB.py = canvasPos.y;
      contextB.dragStartX = utils.dragGetX_(event, contextB);
      contextB.dragStartY = utils.dragGetY_(event, contextB);
      contextB.cancelNextDblclick = false;
      contextB.tarp.cover();
    }
function(event, g, contextB) {
      // prevents mouse drags from selecting page text.
      if (event.preventDefault) {
        event.preventDefault();  // Firefox, Chrome, etc.
      } else {
        event.returnValue = false;  // IE
        event.cancelBubble = true;
      }

      var canvasPos = utils.findPos(g.canvas_);
      contextB.px = canvasPos.x;
      contextB.py = canvasPos.y;
      contextB.dragStartX = utils.dragGetX_(event, contextB);
      contextB.dragStartY = utils.dragGetY_(event, contextB);
      contextB.cancelNextDblclick = false;
      contextB.tarp.cover();
    }
contextB is the same thing as this context object but renamed.