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.

def _write_series(self, workbook, worksheet):
        """
        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.
        """
        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)