public Number getStartValue(int row, int column, int subinterval);

    /**
     * Returns the start value of a sub-interval for a given item.
     *
     * @param rowKey  the row key.
     * @param columnKey  the column key.
     * @param subinterval  the sub-interval.
     *
     * @return The start value (possibly <code>null</code>).
     *
     * @see #getEndValue(Comparable, Comparable, int)
     */
    public Number getStartValue(Comparable rowKey, Comparable columnKey,
                                int subinterval);