Quote Originally Posted by mariposa View Post
In my application I visualize found peaks by labels which are painted on the top part of the plot canvas. Therefore I created a class that inherits QwtPlotItem and I implemented the draw() function accordingly
Guess the implementation of your QwtPlotItem doesn't work together with painter transformations, that are enabled when painting to a paint device with a different resolution than the screen resolution.

But without knowing your code it is impossible to say more,

Uwe