filterDisplayed (filterDisplayed) {
        if (!arguments.length) {
            return this._filterDisplayed;
        }
        this._filterDisplayed = filterDisplayed;
        return this;
    }
Get or set the function that filters options prior to display. By default options
with a value of < 1 are not displayed.
@param {function} [filterDisplayed]
@returns {Function|CboxMenu}
@example
// display all options override the `filterDisplayed` function:
chart.filterDisplayed(function () {
    return true;
});