Refactor axislines nth_coord

[oscargus]

PR summary

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• Plotting related features are demonstrated in an example
• New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines

[anntzer]

Does that really need a public setter? Is there any of the downstream code that could reasonably make use of that functionality? Does the rest of the code even work (eg are caches invalidated properly) if you change nth_coord?

If anything it make make more sense to deprecate direct attribute access to make that immutable. (I'm not sure any of the downstream code even really needs access to that...)

[oscargus]

Good question. The main thing was really to move the common get_nth_coord and then the rest just followed.

There is an example for a private method (replying from the phone...) where it is changed, but I haven't checked more.

You are probably correct that it may make more sense to make it a private attribute. Will have a further look at it.