docs: def vertical_line(self,
                        x: Union[int, float],
                        y1: Union[int, float],
                        y2: Union[int, float],
                        emphasize: bool = False
                        ) -> None:
          """Adds a line from (x, y1) to (x, y2)."""
          y1, y2 = sorted([y1, y2])
          self.vertical_lines.append(_VerticalLine(x, y1, y2, emphasize))