public int getSeriesCount() {
        return this.underlying.getSeriesCount();
    }

    /**
     * Returns the key for a series.
     *
     * @param series  the series index (in the range <code>0</code> to
     *     <code>getSeriesCount() - 1</code>).
     *
     * @return The series key.
     */
    public Comparable getSeriesKey(int series) {
        return this.underlying.getSeriesKey(series);