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