Open Flash Chart, is open source and free to use! You can even get the source code to fiddle with! It was started an maintained by John Glazebrook who has generously kept this project open source. What I love most about this project, is the way it started. I think most projects started this way as well.
Here’s his story
“Yes. Once upon a time I had to deal with a company who sell flash charting components, their component had a bug that I needed fixing, so I emailed them about it asking when it’d be fixed. (Remember that I had paid real money for this software.) They were so incompetent, rude and obnoxious that after three or four weeks of emails I thought to myself “I could learn Flash and Actionscript and write my own charting component, release it as Open Source, host it on sourceforge and build up a community of helpful coders faster than they can fix a single bug.” And that is what I did. And that is why it is free. I guess the moral of the lesson is: don’t piss off your customers.”