PR Summary

Took me a while and a bit of trial and error to figure it out.

The ant example in the doc-string is not that good since for plot, it seems like to points are reordered, so plot([0, 1], [0, 1]) and plot([1, 0], [1, 0]) actually gives the same tick direction. I guess it is correct though, and that plot swaps when creating the line.

One may argue if changing sign or adding/subtracting 180 is the "correct way", but I think that once one realize that it is the angle that one should modify, one can figure out the rest. (Changing sign of the length is also an option, but more limited and I do not think that has any actual advantage...)

Must of course not go into 3.7.2, but if it was only the example, it would probably go into 3.7-doc and so on.

PR Checklist

Documentation and Tests

• Has pytest style unit tests (and pytest passes)
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• New plotting related features are documented with examples.

Release Notes

• New features are marked with a .. versionadded:: directive in the docstring and documented in doc/users/next_whats_new/
• API changes are marked with a .. versionchanged:: directive in the docstring and documented in doc/api/next_api_changes/
• Release notes conform with instructions in next_whats_new/README.rst or next_api_changes/README.rst