I was working on a project lately that involved charts and graphs which had to be interactive, lightweight and somewhat complicated. I looked through quite a lot of different chart APIs, and for some time thought that I’d have to go with Open Flash Chart which is good and simple, but it was Flash. There’s nothing wrong with Flash, but Flash is what makes it more complicated to modify and extend. The standard OFC functionality seemed to work fine for me, but soon I came across Highcharts.
Highcharts is free for personal use, commercial licenses start from $80, which is quite okay. Check out the Highcharts Demo page for some terrific usage examples. Happy charting and thanks for sharing!