code: function renderBack(padding, attrs) {
      var backShape = this.get('backShape');
      var innerBox = this.getBBox();
      // const parent = this.get('parent'); // getParent
      Util.merge(attrs, {
        x: innerBox.minX - padding[3],
        y: innerBox.minY - padding[0],
        width: innerBox.width + padding[1] + padding[3],
        height: innerBox.height + padding[0] + padding[2]
      });
      if (backShape) {
        backShape.attr(attrs);
      } else {
        backShape = this.addShape('rect', {
          zIndex: -1,
          attrs: attrs
        });
      }
      this.set('backShape', backShape);
      this.sort();
      return backShape;
    }
docs: 绘制背景
  @param  {Array} padding 内边距
  @param  {Attrs} attrs 图形属性
  @param  {Shape} backShape 背景图形
  @return {Object} 背景层对象