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