code: function(event, g) {
    var chartPos = utils.findPos(g.canvas_);
    var box = {
      left: chartPos.x,
      right: chartPos.x + g.canvas_.offsetWidth,
      top: chartPos.y,
      bottom: chartPos.y + g.canvas_.offsetHeight
    };
  
    var pt = {
      x: utils.pageX(event),
      y: utils.pageY(event)
    };
  
    var dx = distanceFromInterval(pt.x, box.left, box.right),
        dy = distanceFromInterval(pt.y, box.top, box.bottom);
    return Math.max(dx, dy);
  }
docs: Returns the number of pixels by which the event happens from the nearest
  edge of the chart. For events in the interior of the chart, this returns zero.