promptValue (promptValue) {
        if (!arguments.length) {
            return this._promptValue;
        }
        this._promptValue = promptValue;

        return this;
    }
Controls the default value to be used for
[dimension.filter](https://github.com/crossfilter/crossfilter/wiki/API-Reference#dimension_filter)
when only the prompt value is selected. If `null` (the default), no filtering will occur when
just the prompt is selected.
@param {?*} [promptValue=null]
@returns {*|CboxMenu}