Keep the size of a user defined item

Hello everybody,

i wanna draw a user defined symbol - the shape of a small mobile robo - on a qwtpolarplot with enabled qwtpolarmagnifier.

For the first try i used a qwtpolarmarker with a rectangular qwtsymbol as symbol.
Code:

QwtSymbol( QwtSymbol::Rect, Qt::NoBrush, QPen( QBrush( Qt::blue ), 4, Qt::SolidLine ), QSize( 70, 50 ) )
Unfortunately the symbol is not becoming bigger/smaller when i zoom in/out. Is there an easy way to increase/decrease the size of this rectangular when the user is zooming??

As i understand this the size in painter cord (70;50) is kept constant when i use the zooming function - but i want the real size of the robo (e. g. 1m;0.7m) kept constant. Hope you understand what i try to express.

My second idea was to subclass qwtpolaritem, but i have no idea how i can achieve my goal that the size of the symbol is increased/decreased when i use the mousewheel to zoom in/out.

Another question: is there a possibility to rotate a qwtsymbol so that i can display the 2D-angle of the robo??

Instead of all i could use a qwtcurve to display the shape of the robo (and also the 2D-angle) but this sounds like a quick and dirty solution.

Hope someone can point me in the right direction. ;)

Regards

semeto