round (round) {
        if (!arguments.length) {
            return this._round;
        }
        this._round = round;
        return this;
    }
Set or get the rounding function used to quantize the selection when brushing is enabled.
@example
// set x unit round to by month, this will make sure range selection brush will
// select whole months
chart.round(d3.timeMonth.round);
@param {Function} [round]
@returns {Function|CoordinateGridMixin}