Strang qwtPlot error in qt-designer

Hello,

in qt-designer I created a new gui project, dragged a button and a qwtPlot widget onto the canvas and added some lines of code into mainwindow.cpp (see Attachment 13449, this is a simple qt-creator project which reproduces the described error shown in the screen shot). I expected to see a sine curve but got some strange lines instead (see this screen shot Attachment 13450). These lines change a bit with every new start of this app. I can't figure out what's going on here. What did I do wrong?

My idea is to adapt this example https://de.wikibooks.org/wiki/Qt_f%C...er:_Qwt_nutzen to qt-creators (weird and convoluted to me) way of setting up new gui app projects. (I mean having a class Mainwwindow which inherits from Ui_Mainwindow in the Ui namespace and which takes a setup void setupUi(QMainWindow *MainWindow) function as argument... (see Attachment 13449) Why is this being done this way? Why so complicated?)

My suspicion is that this has to do something with name space mismatch or access restrictions,but I'm not sure...

Thanks

kay