The Excel worksheet reference to the categories for this chart (not
        including the column heading).

def categories_ref(self):
        """
        The Excel worksheet reference to the categories for this chart (not
        including the column heading).
        """
        categories = self._chart_data.categories
        if categories.depth == 0:
            raise ValueError('chart data contains no categories')
        right_col = chr(ord('A') + categories.depth - 1)
        bottom_row = categories.leaf_count + 1
        return "Sheet1!$A$2:$%s$%d" % (right_col, bottom_row)