def build_header_cells_weeks(top, height, columns)
      build_header_cells_parts(columns, top, height,
                               ->(date) { [date.year, date.cweek] },
                               ->(date, week_tuple) { date.year == week_tuple[0] && date.cweek == week_tuple[1] },
                               ->(week_tuple) { "W#{week_tuple[1]}" })