I also have a few questions that I think will help guide the rest of the PR:

1. Should users be able to include an overlay image directly in the call to img.orthoview() instead of having to save the returned OrthoSlicer3D object and call the set_overlay method explicitly? If so, how many of the set_overlay parameters should be exposed in the .orthoview() method?
2. I haven't yet tackled the issue of plotting two 4D volumes. In particular, I'm not sure what to do for the fourth (volume trace) axis. It would likely be very noisy to have both traces plotted at once. We could potentially use a twinx axis instance if desired, but I'd be open to suggestions on what you think would "look best".
3. From what I can tell there is currently no documentation on OrthoSlicer3D or the .orthoview() method in nibabel (except for in the API reference). I (personally) use this feature all the time and would be happy to add some documentation on it (including this new overlay feature), but wanted to confirm that it had not been consciously/purposefully excluded from the docs before moving forward.

1. Should users be able to include an overlay image directly in the call to img.orthoview() instead of having to save the returned OrthoSlicer3D object and call the set_overlay method explicitly? If so, how many of the set_overlay parameters should be exposed in the .orthoview() method?

I don't have strong opinions here. I think following matplotlib conventions for passing up additional parameters is probably going to provide the most expected (if not intuitive) interface.

2. I haven't yet tackled the issue of plotting two 4D volumes. In particular, I'm not sure what to do for the fourth (volume trace) axis. It would likely be very noisy to have both traces plotted at once. We could potentially use a twinx axis instance if desired, but I'd be open to suggestions on what you think would "look best".

twinx looks reasonable to me. In addition, you could think about something where you extend the y-axis up ~2SD for one graph and ~2SD down for the other, so they are mostly non-overlapping. I suspect the only way to find what looks good will be to fiddle with it.

3. From what I can tell there is currently no documentation on OrthoSlicer3D or the .orthoview() method in nibabel (except for in the API reference). I (personally) use this feature all the time and would be happy to add some documentation on it (including this new overlay feature), but wanted to confirm that it had not been consciously/purposefully excluded from the docs before moving forward.

Absolutely, it would be a great addition. And doc updates are welcome during the RC phase.

Hi @rmarkello, just checking in on this. Do you want to aim for a 3.1 release (soon-ish...) or 3.2?

Hey @effigies ! Thanks for the ping, and sorry for the delayed response.

I'm happy to try and get core functionality into 3.1, but am not sure if a full docs addition would be feasible by then. Are you willing to split that into a separate PR, or would you rather I hold off on this 'till 3.2?

Quick responses are beyond me, as well. It would be good to have docs and features together, but I'm also not in a great rush to release if you think you're going to have time. So I'll leave it to you to decide. I'll try to be more prompt in responding, though my time is extremely constrained these days.

Hey @effigies! I think getting this done in the next few weeks is gonna be a bit of a stretch for me, unfortunately... I'd say go ahead and release without this and I'll try and get to it in the next ~month.

Sounds good.

Just checking back in on this one, @rmarkello. Anything I can do to help this along?

Hey @effigies ! Sorry for the radio silence—lots of deadlines over the past several months, though things are looking a bit better in the short term. I think I'll have some time in the coming weeks to work on this (famous last words). I managed to make some minor progress today (I decided to punt combo 4d under/overlay and just TypeError it—happy to discuss pros/cons to that decision if you'd like).

One question re documentation: do you mind if I create a new page that describes the OrthoSlicer3D object? From my reading it doesn't really fit into any of the current pages of the user manual, though I'm happy to take suggestions otherwise!

Sorry again for letting this sit so long.

(I decided to punt combo 4d under/overlay and just TypeError it—happy to discuss pros/cons to that decision if you'd like).

Since it's a data shape (rather than a Python type) issue, I'd go with ValueError. NotImplementedError might also work...

do you mind if I create a new page that describes the OrthoSlicer3D object?

Not at all. It would fit well in https://nipy.org/nibabel/manual.html, if you don't already have a good place in mind.