I can vouch for flot. I've used it successfully on several projects and found it very easy to use. It's built on top of jquery.

It's 100% javascript, free, open source, supports all common browsers, doesn't require internet access, etc.

The only requirement you specify that I'm not sure about is support for pie charts. I've never had to do those, and don't see any on the examples page... so you might check into that if it's a deal breaker.