xAxisOnClick (handler) {
        if (!arguments.length) {
            return this._xAxisOnClick;
        }
        this._xAxisOnClick = handler;
        return this;
    }
Gets or sets the handler that fires when a column tick is clicked in the x axis.
By default, if any cells in the column are unselected, the whole column will be selected,
otherwise the whole column will be unselected.
@param  {Function} [handler]
@returns {Function|HeatMap}