Also, the line object has a method set_dashes() and I wonder if it's a viable option to add a similar method to the spine object.

https://matplotlib.org/stable/gallery/lines_bars_and_markers/line_demo_dash_control.html

(offset, (on_ink, off_ink))

I think this would not be quite right as it seems to imply only a length 2 on-off pattern, whereas any even number should work. The linestyles example explains things pretty well, so possibly a good way to improve here is to just link the example from the docstring?

There is a PR which deals with most of these things that hopefully will be merged for 3.9: #23056

(The discussions are really about what get_linestyle and get_dashes should return, as that is currently inconsistent between classes and unifying it will probably break some code somewhere.)

For the original issue: yes, it is maybe unclear, but correct. Probably it can be clarified a bit further in the PR above, but it does link to the example @rcomer pointed out.

An additional challenge here is that set_dashes only takes the on-off-seq-part so it is not possible to change offset later.

(offset, (on_ink, off_ink))

I think this would not be quite right as it seems to imply only a length 2 on-off pattern, whereas any even number should work. The linestyles example explains things pretty well, so possibly a good way to improve here is to just link the example from the docstring?

I haven't seen this page before. Thanks for sharing. And yes, I think having this page linked in the doc would help as well.