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