Any sufficiently advanced plotting library with an api that can be called externally becomes indistinguishable from a GUI toolkit: https://www.gnu.org/software/guile/docs/guile-tut/tutorial.h...

Not sure if that is the right tutorial, but many years ago in the guile 1.x days I wrote a local visualizer for the data from a particle physics accelerator entirely in Guile and Gnuplot. It was very MVC and used guile as the controller and Gnuplot as the viewer.

Was it stupid? Yes. Did it work better than all the other tools I had at the time? Also yes.