The dialog box in the example quoted above is generated
automatically by Root using the Run Time Type Information (RTTI)
for all classes for which the dictionary is available.
In this example, the dialog box corresponds to TH1::Fit
with 3 parameters.
When clicking on a TH1 object with the right mouse button,
the dialog box is generated on the fly from the RTTI for all the
class member functions that have the string "*MENU*" in the comment
field following the function declaration in the header file.
see http://root.cern.ch/root/html/TH1.h
If you want to create yourself dialog boxes, have a look
at the example in $ROOTSYS/test/guitest.cxx
Rene Brun