RGraph is Free and Open Source Software using the GPL licenseShare RGraph:
RGraph was originally started as a commercial project - however it's now licensed using the GPL license and therefore truly Free and Open Source Software (FOSS).
The GPL license is well known as an Open Source license and means that you can use RGraph for any purpose you wish - non-commercial or commercial. You can include it your own software, modify it and distribute it without asking for permission or paying any fees.
There is nothing that the end user needs to do or buy and there's no requirement for you to contribute back your changes should you make any assuming they're kept private (the GPL does have a Share-alike clause to help preserve the Open Source spirit).
The commercial licensing option
If you don't want to be subject to the GPL, its sharing clauses, its "viral nature" and implications on commercial code there is a commercial licensing scheme available as an alternative. You can read more about the commercial license here.