public Comparable getSeriesKey() {
        return this.seriesKey;
    }
    
    /**
     * Sets the key for the series.
     * 
     * @param key  the key.
     * 
     * @since 1.0.6
     * 
     * @see #getSeriesKey()
     */
    public void setSeriesKey(Comparable key) {
        this.seriesKey = key;
    }
    
    /**
     * Returns the series index.
     *
     * @return The series index.
     * 
     * @see #setSeriesIndex(int)
     * 
     * @deprecated As of 1.0.6, use the {@link #getSeriesKey()} method.
     */
    public int getSeriesIndex() {
        return this.seriesIndex;
    }

    /**
     * Sets the series index.
     *
     * @param index  the series index.
     * 
     * @see #getSeriesIndex()
     * 
     * @deprecated As of 1.0.6, use the {@link #setSeriesKey(Comparable)} 
     *         method.
     */
    public void setSeriesIndex(int index) {
        this.seriesIndex = index;