xAxis (xAxis) {
        if (!arguments.length) {
            if (!this._xAxis) {
                this._xAxis = this._createXAxis();
            }
            return this._xAxis;
        }
        this._xAxis = xAxis;
        return this;
    }
Set or get the x axis used by a particular coordinate grid chart instance. This function is most
useful when x axis customization is required. The x axis in dc.js is an instance of a
{@link https://github.com/d3/d3-axis/blob/master/README.md#axisBottom d3 bottom axis object};
therefore it supports any valid d3 axisBottom manipulation.

**Caution**: The x axis is usually generated internally by dc; resetting it may cause
unexpected results. Note also that when used as a getter, this function is not chainable:
it returns the axis, not the chart,
{@link https://github.com/dc-js/dc.js/wiki/FAQ#why-does-everything-break-after-a-call-to-xaxis-or-yaxis
so attempting to call chart functions after calling `.xAxis()` will fail}.
@see {@link https://github.com/d3/d3-axis/blob/master/README.md#axisBottom d3.axisBottom}
@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]);
@param {d3.axis} [xAxis=d3.axisBottom()]
@returns {d3.axis|CoordinateGridMixin}