[flacon]

Highcharts is my fav and default charting library.

http://www.highcharts.com

Highcharts is not open source but is free to use for non-commercial use. I have paid the $80 to use it on commercial websites and client/managers are always very impressed with its capabilities. I think its well-worth the $$

http://www.highcharts.com/license

[bricestacey]

Highcharts is open source, but it has a restrictive license for commercial use. It is free for non-commercial use (personal, non-profit) and it is also free during development, even commercial apps.

I also highly recommend. It's a pleasure to work with. Sites using it include MixPanel if you want to see it in action.

http://www.highcharts.com/license