filterHandler (filterHandler) {
        if (!arguments.length) {
            return this._filterHandler;
        }
        this._filterHandler = filterHandler;
        return this;
    }
Set or get the filter handler. The filter handler is a function that performs the filter action
on a specific dimension. Using a custom filter handler allows you to perform additional logic
before or after filtering.
@see {@link https://github.com/crossfilter/crossfilter/wiki/API-Reference#dimension_filter crossfilter.dimension.filter}
@example
// the default filter handler handles all possible cases for the charts in dc.js
// you can replace it with something more specialized for your own chart
chart.filterHandler(function (dimension, filters) {
    if (filters.length === 0) {
        // the empty case (no filtering)
        dimension.filter(null);
    } else if (filters.length === 1 && !filters[0].isFiltered) {
        // single value and not a function-based filter
        dimension.filterExact(filters[0]);
    } else if (filters.length === 1 && filters[0].filterType === 'RangedFilter') {
        // single range-based filter
        dimension.filterRange(filters[0]);
    } else {
        // an array of values, or an array of filter objects
        dimension.filterFunction(function (d) {
            for (var i = 0; i < filters.length; i++) {
                var filter = filters[i];
                if (filter.isFiltered && filter.isFiltered(d)) {
                    return true;
                } else if (filter <= d && filter >= d) {
                    return true;
                }
            }
            return false;
        });
    }
    return filters;
});

// custom filter handler
chart.filterHandler(function(dimension, filter){
    var newFilter = filter + 10;
    dimension.filter(newFilter);
    return newFilter; // set the actual filter value to the new value
});
@param {Function} [filterHandler]
@returns {Function|BaseMixin}