code: def _compute_sorted_indices(self):
          sorted_indices = []
          for to_sort in [self.y] + self.x:
              data_w_indices = [(val, i) for (i, val) in enumerate(to_sort)]
              data_w_indices.sort()
              sorted_indices.append([i for val, i in data_w_indices])
          self._yi_sorted = sorted_indices[0]
          self._xi_sorted = sorted_indices[1:]