I tried to figure out if I can use QCustomPlot in my project. Since I don't care what anyone does with my software, I always use the very permissive MIT license and be done with it. From what I've gathered it's not possible for me to use QCustomPlot.
To quote the MIT license Wikipedia entry:
"MIT licensed software can be integrated into GPL software, but not the other way around."
There is also the following thread: