> In laying out the map, an N-body algorithm is run to determine positions based on references between the papers. There are two “forces” involved in the N-body calculation: each paper is repelled from all other papers using an anti-gravity inverse-distance force, and each paper is attracted to all of its references using a spring modelled by Hooke’s law.
However it must have taken them a while to converge for 10^6 particles.
Basically, it's like having a spring between each node (paper) and letting the equilibrium do the rest.