docs: def _add_observation_to_means(self, xj, yj):
          """Update the means without recalculating for the addition of one observation."""
          self._mean_x_in_window = ((self.window_size * self._mean_x_in_window + xj) /
                                    (self.window_size + 1.0))
          self._mean_y_in_window = ((self.window_size * self._mean_y_in_window + yj) /
                                    (self.window_size + 1.0))