'Return the minimum bounding range for all cells containing data.'

def calculate_dimension(self):

    if self._cells:
        rows = set()
        cols = set()
        for (row, col) in self._cells:
            rows.add(row)
            cols.add(col)
        max_row = max(rows)
        max_col = max(cols)
        min_col = min(cols)
        min_row = min(rows)
    else:
        return 'A1:A1'
    return ('%s%d:%s%d' % (get_column_letter(min_col), min_row, get_column_letter(max_col), max_row))
