Returns the same diagram, but mirrored across its diagonal.

def transpose(self) -> 'TextDiagramDrawer':
        """Returns the same diagram, but mirrored across its diagonal."""
        out = TextDiagramDrawer()
        out.entries = {(y, x): _DiagramText(v.transposed_text, v.text)
                       for (x, y), v in self.entries.items()}
        out.vertical_lines = [_VerticalLine(*e)
                              for e in self.horizontal_lines]
        out.horizontal_lines = [_HorizontalLine(*e)
                                for e in self.vertical_lines]
        out.vertical_padding = self.horizontal_padding.copy()
        out.horizontal_padding = self.vertical_padding.copy()
        return out