Force tree transition chaining (incrementally showing nodes and links in a force graph)

Force graph with node-link show up through chaining …… ∆ ∞

Derived from gist 1125997, but now shows the nodes appear incrementally.

This variant also chains the display of the links; links are only shown once their nodes are visible; then they are faded in all at once.

The alternative, which fades in all relevant links one by one (by chaining) for each node which shows up, see gist 2514183.