docs: def _initialize(self):
          """Set up and normalize initial data once input data is specified."""
          self.y_transform = self.y - numpy.mean(self.y)
          self.y_transform /= numpy.std(self.y_transform)
          self.x_transforms = [numpy.zeros(len(self.y)) for _xi in self.x]
          self._compute_sorted_indices()