For most transitions, you don't tell D3 exactly what to do, you just tell it where you want to start and where you want to end. But if you need more control -- e.g., you need an animation to move along a curvy line -- attrTween might do the trick. Based on Mike Bostock's http://bl.ocks.org/mbostock/1705868, mostly tweaked to make it easier for beginners to understand.