I would like to develop a process modeling application in C++. For that, I need a cross-platform UI library with decent graphics features to plot calculation results, and possibly design process diagram. I also need threading API for parallel computations. I think it would be nice to have threading API in the same library, since threading and UI from different libraries might cause conflicts.
Qt is a popular choice but the licensing is tricky. Some tools that I would need like Qt Charts and design tools require to purchase commercial license which is way too expensive.
wxWidgets looks like a good choice but its threading API seems too low-level and confusing.
Please tell me your opinion on the above or can you recommend other libraries which are free and easy to use?