code: function (dx, dy) {
          switch (this.draggable) {
              case 'horizontal':
                  dy = 0;
                  break;
              case 'vertical':
                  dx = 0;
                  break;
          }
  
          var m = this.transform;
          if (!m) {
              m = this.transform = [1, 0, 0, 1, 0, 0];
          }
          m[4] += dx;
          m[5] += dy;
  
          this.decomposeTransform();
          this.dirty(false);
      }
docs: Drift element
  @param  {number} dx dx on the global space
  @param  {number} dy dy on the global space