Write the categories column(s) to *worksheet*. Categories start in
        the first column starting in the second row, and proceeding one
        column per category level (for charts having multi-level categories).
        A date category is formatted as a date. All others are formatted
        `General`.

def _write_categories(self, workbook, worksheet):
        """
        Write the categories column(s) to *worksheet*. Categories start in
        the first column starting in the second row, and proceeding one
        column per category level (for charts having multi-level categories).
        A date category is formatted as a date. All others are formatted
        `General`.
        """
        categories = self._chart_data.categories
        num_format = workbook.add_format({
            'num_format': categories.number_format,
        })
        depth = categories.depth
        for idx, level in enumerate(categories.levels):
            col = depth - idx - 1
            self._write_cat_column(worksheet, col, level, num_format)