RGraph was originally started as a commercial project - however it's now licensed using the MIT license and therefore truly Free and Open Source Software (FOSS).
The MIT 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. Basically: You can do what you like with it!