seriesSort (sortFunction) {
        if (!arguments.length) {
            return this._seriesSort;
        }
        this._seriesSort = sortFunction;
        this._resetChildren();
        return this;
    }
Get or set a function to sort the list of series by, given series values.
@see {@link https://github.com/d3/d3-array/blob/master/README.md#ascending d3.ascending}
@see {@link https://github.com/d3/d3-array/blob/master/README.md#descending d3.descending}
@example
chart.seriesSort(d3.descending);
@param {Function} [sortFunction=d3.ascending]
@returns {Function|SeriesChart}