addFilterHandler (addFilterHandler) {
        if (!arguments.length) {
            return this._addFilterHandler;
        }
        this._addFilterHandler = addFilterHandler;
        return this;
    }
Set or get the add filter handler. The add filter handler is a function that adds a filter to
the chart's filter list. Using a custom add filter handler allows you to change the way filters
are added or perform additional work when adding a filter, e.g. when using a filter server other
than crossfilter.

The handler should return a new or modified array as the result.
@example
// default add filter handler
chart.addFilterHandler(function (filters, filter) {
    filters.push(filter);
    return filters;
});

// custom filter handler (no-op)
chart.addFilterHandler(function(filters, filter) {
    return filters;
});
@param {Function} [addFilterHandler]
@returns {Function|BaseMixin}