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