Flockr generates digraphs out of your flickr contacts. This is what has triggered me to go off the straight & narrow path of low level programming into hacks with Graph networks and x-mixed-replace. Anyway, what is done is done - take a look at a canned demo if you are on Firefox 1.5 and above (can't host cgi scripts).
The really hard part of the above code comes out of the simple fact that HTML Canvas is totally pixel based. You cannot attach an event to a small circle drawn as lines & figures. The code inside graph.js has a class called NodeHandler which sort of implements event masks and handlers for a node I've drawn on canvas. The code uses a hashed bucket to determine which all nodes it needs to iterate over, rather than loop over every node in the graph for every mouse move - harsh lessons learnt from dotgnu's winforms. It works better than I'd expected of Javascript and certainly seems snappy enough.
Now, I'm off hacks for a while ... time to vent some energy on real code.
--An empty canvas has more potential for greatness than any painting.