This does not work,
Finally by launching the compilation nothing happens, the chosen value does not appear in the lineEdit.

I would like to start by taking a value that we choose in the .cpp and then with a QString, text () or setText () or whatever, the chosen value appears in the lineEdit.
Once the value appears, it is necessary to link the lineEdit and the QwtThermo so that the value in the lineEdit also appears in the QwtThermo (50 in lineEdit so 50 in QwtThermo).

I tested your program but nothing appears in lineEdit, even choosing 50 as a value.

Do you understand what I've said, do you have another solution ?