xBorderRadius (xBorderRadius) {
        if (!arguments.length) {
            return this._xBorderRadius;
        }
        this._xBorderRadius = xBorderRadius;
        return this;
    }
Gets or sets the X border radius.  Set to 0 to get full rectangles.
@param  {Number} [xBorderRadius=6.75]
@returns {Number|HeatMap}