Unfortunately I think subfigures are too conflicting with proplot's paradigms and redundant with proplot features to support. Here are my reasons:

1. Subfigures seem to primarily be an extension of GridSpecFromSubplotSpec, which proplot does not explicitly support. Instead, if you want certain subplots to appear "grouped together", you can create a GridSpec with vector-input wspace and hspace (e.g., the above example could be something like GridSpec(ncols=3, nrows=3, wspace=(1, 3))).
2. Subfigures seem to be potentially useful for making subplots with different "styles", but SubplotGrid combined with the format command makes it pretty straightforward to style individual subplots or subplot groups differently.
3. It seems ambiguous how a-b-c labels should be assigned for subplots inside subfigures. This would be a whole can of worms we have to figure out.
4. It seems that unlike GridSpecFromSubplotSpec, the subplot edges in each "subfigure" can be misaligned. But I don't really see how this is useful... it just looks kind of messy tbh.
5. It would be useful to "concatenate" or save individual subfigures, but matplotlib does not seem to allow this (I might implement this in proplot some day -- Add function for "concatenating" figures #203).

So, it seems like most subfigure features can be reproduced in proplot already. Maybe I'll add a note in the documentation about how SubFigure and GridSpecFromSubplotSpec are not officially supported.

Let me know if there's any subfigure use cases I'm missing here.

One last "subfigure" feature that I can see being useful is adding shading behind different groups of subplots (like in your example). Come to think of it I've actually had to do this before. But this could definitely fall under the umbrella of the format command. I'll add it to my to-do list. Could be something like highlightcolor or backgroundcolor.

• Seems logical. Maybe adding into the documentation might be helpful. Thanks!
• Regarding concatenation of the figures is of particular interest to me. There are several instances where I had generated several figures but lacked a straightforward method to concatenate all of them into a single figure.