sortBy (sortBy) {
        if (!arguments.length) {
            return this._sortBy;
        }
        this._sortBy = sortBy;
        return this;
    }
Get or set sort-by function. This function works as a value accessor at row level and returns a
particular field to be sorted by.
@example
chart.sortBy(function(d) {
    return d.date;
});
@param {Function} [sortBy=identity function]
@returns {Function|DataTable}