@property
    def _categories_ref(self):
        end_row_number = len(self._categories) + 1
        return "Sheet1!$A$2:$A$%d" % end_row_number
The Excel worksheet reference to the categories for this series.
dataset/ETHPy150Open scanny/python-pptx/pptx/chart/data.py/_SeriesData._categories_ref