xAxis (xAxis) {
        if (!arguments.length) {
            return this._xAxis;
        }
        this._xAxis = xAxis;
        return this;
    }
Get or sets the x axis for the row chart instance.
See the {@link https://github.com/d3/d3-axis/blob/master/README.md d3.axis}
documention for more information.
@param {d3.axis} [xAxis]
@example
// customize x axis tick format
chart.xAxis().tickFormat(function (v) {return v + '%';});
// customize x axis tick values
chart.xAxis().tickValues([0, 100, 200, 300]);
// use a top-oriented axis. Note: position of the axis and grid lines will need to
// be set manually, see https://dc-js.github.io/dc.js/examples/row-top-axis.html
chart.xAxis(d3.axisTop())
@returns {d3.axis|RowChart}