xAxisPaddingUnit (unit) {
        if (!arguments.length) {
            return this._xAxisPaddingUnit;
        }
        this._xAxisPaddingUnit = unit;
        return this;
    }
Set or get x axis padding unit for the elastic x axis. The padding unit will determine which unit to
use when applying xAxis padding if elasticX is turned on and if x-axis uses a time dimension;
otherwise it is ignored.

The padding unit should be a
[d3 time interval](https://github.com/d3/d3-time/blob/master/README.md#self._interval).
For backward compatibility with dc.js 2.0, it can also be the name of a d3 time interval
('day', 'hour', etc). Available arguments are the
[d3 time intervals](https://github.com/d3/d3-time/blob/master/README.md#intervals d3.timeInterval).
@param {String} [unit=d3.timeDay]
@returns {String|CoordinateGridMixin}