docs: def _calculate_average(self, points):
          """Calculate the arithmetic mean of the points x and y coordinates
             seperately.
          """
          assert len(self.theta) == len(points), \
              "points has length %i, but should have length %i" % \
              (len(points), len(self.theta))
          new_point = {'x': 0, 'y': 0, 'time': 0}
          for key in new_point:
              new_point[key] = self.theta[0] * points[0][key] + \
                  self.theta[1] * points[1][key] + \
                  self.theta[2] * points[2][key]
          return new_point