I worked on a similar little project about 10 years ago. We called it 300 Lines. It was a shared public canvas which displayed only the last 300 lines drawn.

We used socket.io to sync between the client and a NodeJS backend. (The whole thing was really an experiment to get to grips with the then-nascent Node v0.4)

Of course it was horribly abused, and we took it down after only a few days.