code: def y_values_ref(self, series):
          
          top_row = self.series_table_row_offset(series) + 2
          bottom_row = top_row + len(series) - 1
          return "Sheet1!$B$%d:$B$%d" % (top_row, bottom_row)
docs: The Excel worksheet reference to the Y values for this chart (not
          including the column label).