docs: function(x1, y1, x2, y2) {
          if (arguments.length === 1) y1 = x2 = y2 = x1;
          this._bx1 = x1;
          this._bx2 = x2;
          this._by1 = y1;
          this._by2 = y2;
          this.trigger("BoundaryOffset");
          this._calculateMBR();
          return this;
      }