public double getXValue(int series, int item);
    
    /**
     * Returns the y-value for an item within a series.
     *
     * @param series  the series index (in the range <code>0</code> to 
     *     <code>getSeriesCount() - 1</code>).
     * @param item  the item index (in the range <code>0</code> to 
     *     <code>getItemCount(series)</code>).
     *
     * @return The y-value (possibly <code>null</code>).
     */
    public Number getY(int series, int item);