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);