What is QwtSampleThread
My mistake ... It's QwtSamplingThread and not QwtSampleThread

I don't know what you mean by 'time-dependent', of course, the calculation takes some time
I had a look at my post and do understand the confusion. What I wanted to know is if time is the independent variable (time vs dependent_variable)

I am still no clear why you need the help of a separate thread for calculation.