Adds a line from (x1, y) to (x2, y).

def horizontal_line(self,
                        y: Union[int, float],
                        x1: Union[int, float],
                        x2: Union[int, float],
                        emphasize: bool = False
                        ) -> None:
        """Adds a line from (x1, y) to (x2, y)."""
        x1, x2 = sorted([x1, x2])
        self.horizontal_lines.append(_HorizontalLine(y, x1, x2, emphasize))