rowOrdering (rowOrdering) {
        if (!arguments.length) {
            return this._rowOrdering;
        }
        this._rowOrdering = rowOrdering;
        return this;
    }
Get or set a comparator to order the rows.
Default is {@link https://github.com/d3/d3-array#ascending d3.ascending}.
@param  {Function} [rowOrdering]
@returns {Function|HeatMap}