order (order) {
        if (!arguments.length) {
            return this._order;
        }
        this._order = order;
        return this;
    }
Get or set the function that controls the ordering of option tags in the
cbox menu. By default options are ordered by the group key in ascending
order.
@param {Function} [order]
@returns {Function|CboxMenu}
@example
// order by the group's value
chart.order(function (a,b) {
    return a.value > b.value ? 1 : b.value > a.value ? -1 : 0;
});