mbostock/d3 — pull request #573 by mbostock (2012-02-26 23:59:10+00:00)
Title: Inheritance for transition.each.

If you use d3.transition(selection) within the context of a transition.each, the
created transition will automatically inherit the delay, duration and easing of
the parent transition. Likewise, if you use d3.transition(selection) within the
context of a selection.each, it simply returns the passed-in selection with no
transition at all. This makes it easier to create components, such as the axis,
that support automatic transitions.