Yeah, haha funnily enough I'm a huge fan of both C4 and plantuml. We actually do have an experimental 'software map' feature, turned off in settings by default, which draws a hierarchical / C4-based architecture diagram of your codebase (even highlighting the nodes which changed in the PR!) Warning, it is a little rough around the edges at the moment.

As far as plantuml/graphviz/mermaid/similar UML tools, it's really cool stuff, I agree. My issue is that those tools are mainly built to render static diagrams (output to svg, png, etc.) Some eg mermaid are perhaps extensible for interactivity, but it's a little wonky. I think a lot of the value of whiteboard comes from the interactivity -- being able to click on a node to see the underlying code, being able to go through a guided walkthrough of a given flow, etc.

But using their UML authoring format is certainly worth looking into, since the LLMs are already so well-versed in it. i'll put some thought into this!