I've been using the Google Chart Tools (http://code.google.com/apis/charttools/index.html) and have been pretty happy with it. Haven't used it on a major project though, so your mileage may vary.
A lot of these charting libraries use javascript or flash which is fine for user-specific data, but unsuitable for public data.
I need charts to be jpg/gif/png so people can easily copy them to blogs/forums/facebook. If I use javascript or flash charts I'll lose a huge amount of viral traffic.