Adds a line from (x, y1) to (x, y2).

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))