Uwe,

Yes 20000 curves it is too much but I'm exploring QWT and after that I should decide how to design the application.
There is a small free software that can plot more curves with quite good perfomance SEISEE https://mail.dmng.ru/freeware/?lang=en
I can't understand how it works but if you are interested to test it I could find some data to use in SEISEE