docs: function Single(axisModel, ecModel, api) {
  
      /**
       * @type {string}
       * @readOnly
       */
      this.dimension = 'single';
  
      /**
       * Add it just for draw tooltip.
       *
       * @type {Array.<string>}
       * @readOnly
       */
      this.dimensions = ['single'];
  
      /**
       * @private
       * @type {module:echarts/coord/single/SingleAxis}.
       */
      this._axis = null;
  
      /**
       * @private
       * @type {module:zrender/core/BoundingRect}
       */
      this._rect;
  
      this._init(axisModel, ecModel, api);
  
      /**
       * @type {module:echarts/coord/single/AxisModel}
       */
      this.model = axisModel;
  }