Update 2018: 7 years after I bookmarked this graphing library, it is still one of the best
Recently (2018) I had a project to complete on a strict deadline. A Highcharts license was already available. It was really a no brainer. Although I typically prefer open source, none of the open source libraries I’ve found so far are as good, complete and/or as fast as Highcharts. And as the power of the library was known to management, I really needed to make sure an open source library would be really up to that game. Pricing starts at $480 these days.
Originally I wrote:
Cool, fast (SVG based) visualisation library for Javascript. Not completely free though: “Do you want to use Highcharts for a personal or non-profit project? Then you can use Highchcarts for free under the Creative Commons Attribution-NonCommercial 3.0 License. Single websites costs 80USD.”