public Number getPercentComplete(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) {
                result = sub.getPercentComplete();
            }
        }
        return result;