Where necessary, scale graph to fit its canvas.

o Scaling happens at the "end" of the force layout, when movements are already supposed to be imperceptible to the user.
  For a smoother effect it would be better to rescale several time during the course of the force layout
  (but not every tick, because this would make people feel ill)
o Currently scaling doesn't take account of full-screen events. Will investigate how to join them up.