When are bound variables initialized?

FROM : Michael Lutz
DATE : Tue Apr 12 20:34:48 2005

Hi,

in one NIB I have two windows: One main and one Chart window. Those two
windows are controlled by one 'AppController' class. The Chart window
has two radio buttons on it, where the selectedValue is bound to 
NSMutableString    *selectedXAxisValue. When I show the Chart window the
first time selectedValue is nil and none of the buttons is selected.
So my question is: When is selectedValue initialized the first time and
how can I initialize it up-front (so one button is selected)?

Any hints, pointers to documentation are very welcome :).

Cheers,
  Michael