I didn't try anything yet because I don't have any clue what to do.
Indeed, it is possible to use QProgressBar - this was my first intention. But I don't know how to place this widget on top of my qwt-plot-widget. That is my problem.

Furthermore I don't really understand what you want to tell me in your post about a progress widget with Qwt plot as parent. At least, I still don't know how to display the progress bar.