boxOnClick (handler) {
        if (!arguments.length) {
            return this._boxOnClick;
        }
        this._boxOnClick = handler;
        return this;
    }
Gets or sets the handler that fires when an individual cell is clicked in the heatmap.
By default, filtering of the cell will be toggled.
@example
// default box on click handler
chart.boxOnClick(function (d) {
    var filter = d.key;
    events.trigger(function () {
        _chart.filter(filter);
        _chart.redrawGroup();
    });
});
@param  {Function} [handler]
@returns {Function|HeatMap}