如何在自定义JointJS元素中设置路径缩放比例？

var CustomElement = joint.dia.Element.extend({
  markup: '<g class="rotatable"><g class="scalable"><path/></g></g>',
  defaults: joint.util.deepSupplement({
    type: 'custom.Element',
    attrs: {
      path: {
        d: 'M0 0 L100 0 L100 100 L0 100 Z',
        fill: 'blue'
      }
    },
    scale: 0.5
  }, joint.dia.Element.prototype.defaults),
  initialize: function() {
    joint.dia.Element.prototype.initialize.apply(this, arguments);
    this.listenTo(this, 'change:scale', this.updateScale);
  },
  updateScale: function() {
    this.$('path').attr('transform', 'scale(' + this.get('scale') + ')');
  }
});

var element = new CustomElement({
  position: { x: 50, y: 50 },
  size: { width: 100, height: 100 },
  scale: 0.5
});

var graph = new joint.dia.Graph();
graph.addCells([element]);

var paper = new joint.dia.Paper({
  el: document.getElementById('paper'),
  model: graph,
  width: 600,
  height: 400
});