code: def _write_series(self, workbook, worksheet):
          
          col_offset = self._chart_data.categories.depth
          for idx, series in enumerate(self._chart_data):
              num_format = (
                  workbook.add_format({'num_format': series.number_format})
              )
              series_col = idx + col_offset
              worksheet.write(0, series_col, series.name)
              worksheet.write_column(1, series_col, series.values, num_format)
docs: Write the series column(s) to *worksheet*. Series start in the column
          following the last categories column, placing the series title in the
          first cell.