How to properly use QPainter in custom Widgets (inherits from QWidget).

[Exxtay]

I'm on the way to create my interpretation of Nassi-Shneiderman diagram.
It's like my "exam" what i choose to myself to prove that i learned at least basics of Qt programming.
Nassi-Shneiderman diagram looks like follows: http://www.thern.org/projects/nassi-schneiderman/ProblemSolvingTechniques_files/ns-example2.gif

And i've got little problem:
When i use QPainter to draw lines for multiple choise statement for example, i cannot force drawed lines stick to top corners and middle of bottom of widget, while i resizing the widget.

Probably the way to solve that problem is use Widget Resize Event, but how to do that properly? Or here is a better way to solve that problem?