seriesAccessor (accessor) {
        if (!arguments.length) {
            return this._seriesAccessor;
        }
        this._seriesAccessor = accessor;
        this._resetChildren();
        return this;
    }
**mandatory**

Get or set accessor function for the displayed series. Given a datum, this function
should return the series that datum belongs to.
@example
// simple series accessor
chart.seriesAccessor(function(d) { return "Expt: " + d.key[0]; })
@param {Function} [accessor]
@returns {Function|SeriesChart}