I think this is an interesting tool, and it's very easy to use. I had a couple of thoughts:

1)What does the withdrawal rate measure? I took it to mean the % of your portfolio value needed to cover your annual expenses after retirement. Might be worth clarifying this somewhere.

2) Would be cool to see a similar chart for annual rate of return (if for nothing more than to drive home the importance of savings rate).

Either way, I think it does a great job of highlighting just how big an impact little changes can have.