What solution would you suggest?
Matplotlib depends on one (random) backend and I don't think it makes much difference to depend on another one instead, which then drags in other dependencies.

In theory, we could add virtual provides to all backend subpackagers, e.g.:

Provides: python[23]-matplotlib-backend

and then add a hard dependency on it to the main package:

Requires: python[23]-matplotlib-backend = %{version}-%{release}

We could also favour the tk backend by using

Recommends: python[23]-matplotlib-%{backend_subpackage} = %{version}-%{release}

I'll have to test how well this works in practice.

The default backend is also written in the default configuration file.
IIRC, it is not possible to pick a random, existing backend because of that.

Or did that change recently?

No, I haven't changed that. I imagine it's possible to make it work like this:

if just one backend is installed, set it in matplotlibrc
else (more than one is installed):
    if tk backend is present, set tk in matplotlibrc
    else pick the last one installed

I'm not sure it's worth the effort, though.

Basically, I meant to say: patches welcome.

This package has changed maintainer in the Fedora.
Reassigning to the new maintainer of this component.