code: def specify_data_set(self, x_input, y_input, sort_data=False):
          if sort_data:
              xy = sorted(zip(x_input, y_input))
              x, y = zip(*xy)
              x_input_list = list(x_input)
              self._original_index_of_xvalue = [x_input_list.index(xi) for xi in x]
              if len(set(self._original_index_of_xvalue)) != len(x):
                  raise RuntimeError('There are some non-unique x-values')
          else:
              x, y = x_input, y_input
          self.x = x
          self.y = y