myTransition.attrTween( name, tweenCalculator ) in D3.js

Transition an attribute between two values.

Arguments

name	type	description
name	String	Name of the attribute to transition.
tweenCalculator	Function	Invoked for each element in the transition selection at the start of the transition, passed the current datum, index, and current (pre-transition) attribute value, with the `this` context as the current DOM element. This function must then return a tween interpolator function to use during the transition; the interpolator will be passed parameterized time in the domain `[0,1]` and must return the desired attribute value.