Cartopy was once an optional dependency, so the plots also worked without it. But that feature somehow disappeared on the current dev, now it is not working without cartopy anymore. That should be fixed.
It might be helpful to have a look at the changes from commit https://github.com/openego/eTraGo/commit/45c2d00332aab52c0bdd8b17d63f0705a4637332
Cartopy was once an optional dependency, so the plots also worked without it. But that feature somehow disappeared on the current dev, now it is not working without cartopy anymore. That should be fixed. It might be helpful to have a look at the changes from commit https://github.com/openego/eTraGo/commit/45c2d00332aab52c0bdd8b17d63f0705a4637332