Hi Helen,
Hopefully this reply isn't a moot point by now.
I've modified your code so that the initial progress is displayed:
You'll still need to work on the transitions and tween. Looking at it, in attrTween, d, i, a are passed to functions. Data, index, attributes. The data will new, so you can't use it for interpolation. Mike Bostock stores the old values on the DOM element for this reason (look for this._current).
If you haven't already, I would look at the General Update Patterns to get a better understanding of how data is bound to DOM elements.
And as a shameless self-plug, I just recently wrote up some examples on arc tweening. Hopefully you find my examples helpful:
Please let us know if you need any more help!
CHeers,
al