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.
The withdrawal rate is a little tricky. The withdrawal rate is the percentage of your retirement nest egg you will be spending each year. If you decrease your withdrawal rate you will need to save a bigger lump sum to compensate for the smaller amount of income you are generating. Therefore it will take longer for you to retire.
http://www.mrmoneymustache.com/2012/01/13/the-shockingly-sim...
If you are interested in the math behind the calculator see my blog post about it here:
http://blog.networthify.com/math/the-math-behind-the-calcula...