valueSort (sortFunction) {
        if (!arguments.length) {
            return this._valueSort;
        }
        this._valueSort = sortFunction;
        this._resetChildren();
        return this;
    }
Get or set a function to sort each series values by. By default this is the key accessor which,
for example, will ensure a lineChart series connects its points in increasing key/x order,
rather than haphazardly.
@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
// Default value sort
_chart.valueSort(function keySort (a, b) {
    return d3.ascending(_chart.keyAccessor()(a), _chart.keyAccessor()(b));
});
@param {Function} [sortFunction]
@returns {Function|SeriesChart}