public Number getEndValue(Comparable rowKey, Comparable columnKey,
                              int subinterval) {
        Number result = null;
        int row = getRowIndex(rowKey);
        TaskSeries series = (TaskSeries) this.data.get(row);
        Task task = series.get(columnKey.toString());
        if (task != null) {
            Task sub = task.getSubtask(subinterval);
            if (sub != null) {
                TimePeriod duration = sub.getDuration();
                result = new Long(duration.getEnd().getTime());
            }
        }
        return result;
    }

    /**
     * Returns the percentage complete value of a sub-interval for a given item.
     *
     * @param row  the row index (zero-based).
     * @param column  the column index (zero-based).
     * @param subinterval  the sub-interval.
     *
     * @return The percent complete value (possibly <code>null</code>).
     */
    public Number getPercentComplete(int row, int column, int subinterval) {
        Comparable rowKey = getRowKey(row);
        Comparable columnKey = getColumnKey(column);
        return getPercentComplete(rowKey, columnKey, subinterval);