queueboard dependency graph

Stream: mathlib4

[Johan Commelin]

I think it would be useful if the dependency graph had a toggle to hide clusters that don't have any PR that is on the queue.

Also, PRs without dependencies are currently all green. But maybe only the ones that are on the queue should be green.

[Bryan Gin-ge Chen]

I also had been thinking of making a less "jumpy" version with e.g. one of the non-dynamic layouts here; I'll get to this and other frontend tweaks eventually but probably after the new backend is in place.