Is there a piece of software in extras-devel that uses both matplotlib and tcl/tk for something else than displaying stand-alone graphs from matplotlib ? If not I see no reason to add the compiled-in backend that will never be used.

Also I would like to keep the dependencies to bare minimum (which is why it only has compiled in support for GTK [Qt support needs no compilation]).

I wonder about the dependencies though; can we leave out tk as dependency if we have compiled in support for it (at the very least we must change the default backend to GTK if we don't hard-depend on tk) I guess that would need a bunch of experimentation with a reliable way to rollback to a clean state (reflashing is probably easiest and fastest but a pain if you wish to use your device for something else than development only).