code: function (dim, scale, extent) {
  
      /**
       * Axis dimension. Such as 'x', 'y', 'z', 'angle', 'radius'.
       * @type {string}
       */
      this.dim = dim;
  
      /**
       * Axis scale
       * @type {module:echarts/coord/scale/*}
       */
      this.scale = scale;
  
      /**
       * @type {Array.<number>}
       * @private
       */
      this._extent = extent || [0, 0];
  
      /**
       * @type {boolean}
       */
      this.inverse = false;
  
      /**
       * Usually true when axis has a ordinal scale
       * @type {boolean}
       */
      this.onBand = false;
  }
docs: Base class of Axis.
  @constructor