code: def categories_ref(self):
          
          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)
docs: The Excel worksheet reference to the categories for this chart (not
          including the column heading).