public Number getStartValue(Comparable series, Comparable category) {
        int seriesIndex = getSeriesIndex(series);
        if (seriesIndex < 0) {
            throw new UnknownKeyException("Unknown 'series' key.");
        }
        int itemIndex = getColumnIndex(category);
        if (itemIndex < 0) {
            throw new UnknownKeyException("Unknown 'category' key.");
        }
        return getStartValue(seriesIndex, itemIndex);
    }

    /**
     * Returns the start data value for one category in a series.
     *
     * @param series  the required series (zero based index).
     * @param category  the required category.
     *
     * @return The start data value for one category in a series 
     *         (possibly <code>null</code>).
     *         
     * @see #getStartValue(Comparable, Comparable)
     */
    public Number getStartValue(int series, int category) {

        // check arguments...
        if ((series < 0) || (series >= getSeriesCount())) {
            throw new IllegalArgumentException(
                "DefaultIntervalCategoryDataset.getValue(): "
                + "series index out of range.");
        }

        if ((category < 0) || (category >= getCategoryCount())) {
            throw new IllegalArgumentException(
                "DefaultIntervalCategoryDataset.getValue(): "
                + "category index out of range.");
        }

        // fetch the value...
        return this.startData[series][category];