public Number getPercentComplete(Comparable rowKey, Comparable columnKey) {
        Number result = null;
        int row = getRowIndex(rowKey);
        TaskSeries series = (TaskSeries) this.data.get(row);
        Task task = series.get(columnKey.toString());
        if (task != null) {
            result = task.getPercentComplete();
        }
        return result;