This is fantastic.
I don't understand why this did't go bigger on HN.
(Maybe because missing shapes?)
Might consider posting to Product.
Let me know if you do and I'll add my voice (jv at vip dot ie).
I love this and been looking for something like this! Thanks for creating it.
Very cool! Another application that is great for collaborative prototyping is precursor [1]
https://precursorapp.com