code: def _populate_worksheet(self, workbook, worksheet):
          
          self._write_categories(workbook, worksheet)
          self._write_series(workbook, worksheet)
docs: Write the chart data contents to *worksheet* in category chart
          layout. Write categories starting in the first column starting in
          the second row, and proceeding one column per category level (for
          charts having multi-level categories). Write series as columns
          starting in the next following column, placing the series title in
          the first cell.