xUnits (xUnits) {
        if (!arguments.length) {
            return this._xUnits;
        }
        this._xUnits = xUnits;
        return this;
    }
Set or get the xUnits function. The coordinate grid chart uses the xUnits function to calculate
the number of data projections on the x axis such as the number of bars for a bar chart or the
number of dots for a line chart.

This function is expected to return a Javascript array of all data points on the x axis, or
the number of points on the axis. d3 time range functions [d3.timeDays, d3.timeMonths, and
d3.timeYears](https://github.com/d3/d3-time/blob/master/README.md#intervals) are all valid
xUnits functions.

dc.js also provides a few units function, see the {@link units Units Namespace} for
a list of built-in units functions.

Note that as of dc.js 3.0, `units.ordinal` is not a real function, because it is not
possible to define this function compliant with the d3 range functions. It was already a
magic value which caused charts to behave differently, and now it is completely so.
@example
// set x units to count days
chart.xUnits(d3.timeDays);
// set x units to count months
chart.xUnits(d3.timeMonths);

// A custom xUnits function can be used as long as it follows the following interface:
// units in integer
function(start, end) {
     // simply calculates how many integers in the domain
     return Math.abs(end - start);
}

// fixed units
function(start, end) {
     // be aware using fixed units will disable the focus/zoom ability on the chart
     return 1000;
}
@param {Function} [xUnits=units.integers]
@returns {Function|CoordinateGridMixin}