Mermaid.js is a great tool to make diagrams in plain text, I use it a lot and I wanted to have a way to see previews of the code I was writing. There are some options to do that but they require to have a mermaid-cli installed, which requires nodejs as well. Emacs has a built-in webkit browser, in case it was compiled with --with-xwidgets flag, and mermaid run on js so it should be possible to just run the code I want in the browser and see it there.
