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:]