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.

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