code: def ess(self):
          
          _, y_vals, values, colors = self.labels_ticks_and_vals()
          for y, value, color in zip(y_vals, values, colors):
              if value.ndim != 2 or value.shape[0] < 2:
                  yield y, None, color
              else:
                  yield y, _get_ess(value), color
docs: Get effective n data for the variable.