code: function (ecModel, api) {
  
          /**
           * @private
           * @type {module:echarts/data/Tree}
           */
          this._oldTree;
  
          /**
           * @private
           * @type {module:zrender/container/Group}
           */
          this._mainGroup = new graphic.Group();
  
          /**
           * @private
           * @type {module:echarts/componet/helper/RoamController}
           */
          this._controller = new RoamController(api.getZr());
  
          this._controllerHost = {target: this.group};
  
          this.group.add(this._mainGroup);
      }
docs: Init the chart
  @override
  @param  {module:echarts/model/Global} ecModel
  @param  {module:echarts/ExtensionAPI} api