[vtkusers] orientation axes in the corner

[Matt Schmiermund]

I am looking to add some orientation axes in the lower
left hand corner of a vtkRenderWindow.  I have used
the vtkAxes class to add axes that are in the middle
and move with the other actors. I want my axes to
rotate, but not to pan or zoom with the other actors. 
I saw some questions about this in the archives and
those answers have pointed me in the right direction,
but now I have a few questions of my own.

I have set up two layers, one for the axes and the
other for the main actor. 

How do I position the axes so they are in the lower
left corner of the render window? I was hoping to see
something that would allow me to position an actor
based on display coordinates, but I didn't find
anything in the documentation.

I know I need to subclass vtkInteractorStyle, but how
do I make it apply certain actions to only certain
actors?

If anyone could post some code snippets, or even point
me to some other reading I might have missed I would
appreciate it.

-Matt Schmiermund