plot only subset of the data

[Uwe]

Could you direct me to an example, I could not figure out how this could be implemented.

Qwt itsself is full of examples. Look at the classes in qwt_data.h, or in the simple example. ( Passing double arrays also creates an internal QwtData object ).

If I were in your shoes I would implement a data object, that has a pointer to your original data structure holding your samples. Then you can add filters like "every 10th point" or whatever, and implement these filters easily in the virtual methods of QwtData.
In my example YourData::x(i) would return the x value from the sample 10*i from your structure.

Note, that you have to implement YourData::copy(). Here you have to copy the API - not the data itsself!

Uwe