implemented in Boost. I am replacing this with a custom implementation of
a constrained stress-majorization algorithm.
The stress-majorization algorithm is more robust and has better convergence
characteristics than Kamada-Kawai, and also simple constraints can be placed
on node position (for example, to enforce downward-pointing edges, non-overlap constraints, or cluster constraints).
Another big advantage is that we no longer need Boost.
I've tested the basic functionality, but I have yet to properly handle
disconnected graphs or to properly scale the resulting layout.
This commit also includes significant refactoring... the quadratic program solver - libvpsc (Variable Placement with Separation Constraints) has been moved to src/libvpsc and the actual graph layout algorithm is in libcola.
(bzr r1394)