x (xScale) {
        if (!arguments.length) {
            return this._x;
        }
        this._x = xScale;
        this._xOriginalDomain = this._x.domain();
        this.rescale();
        return this;
    }
**mandatory**

Get or set the x scale. The x scale can be any d3
{@link https://github.com/d3/d3-scale/blob/master/README.md d3.scale} or
{@link https://github.com/d3/d3-scale/blob/master/README.md#ordinal-scales ordinal scale}
@see {@link https://github.com/d3/d3-scale/blob/master/README.md d3.scale}
@example
// set x to a linear scale
chart.x(d3.scaleLinear().domain([-2500, 2500]))
// set x to a time scale to generate histogram
chart.x(d3.scaleTime().domain([new Date(1985, 0, 1), new Date(2012, 11, 31)]))
@param {d3.scale} [xScale]
@returns {d3.scale|CoordinateGridMixin}