docs: function () {
          var axisExtent = this._extent;
          var dataExtent = this.scale.getExtent();
  
          var len = dataExtent[1] - dataExtent[0] + (this.onBand ? 1 : 0);
          // Fix #2728, avoid NaN when only one data.
          len === 0 && (len = 1);
  
          var size = Math.abs(axisExtent[1] - axisExtent[0]);
  
          return Math.abs(size) / len;
      }