Very good for 500 lines of code :)! Thanks for sharing. O(n^2) for n-body simulation could hurt performance for the larger graph, i guess. But it should work really well for smaller ones.

BTW this one by Lars Rönnbäck is also very impressive: http://js1k.com/2010-first/demo/618 - press any key to add nodes. It's only 1016 bytes. Amazing.