rows (rows) {
        if (!arguments.length) {
            return this._rows;
        }
        this._rows = rows;
        return this;
    }
Gets or sets the values used to create the rows of the heatmap, as an array. By default, all
the values will be fetched from the data using the value accessor.
@param  {Array<String|Number>} [rows]
@returns {Array<String|Number>|HeatMap}