No picture with imagesc

[Hermann Flaschka]

I am running Octave 2.9.9 on a Mac.  When I try to use imagesc,  
nothing happens. No picture,
no popup, no error message.  Ordinary graphs ( like plot(x,y) ) work  
fine.  Does imagesc require
graphics packages that would have to be installed separately?  I  
don't know if this is relevant,
but commands like "gset term postscript" or "gset term x11" that  
worked with my older version
of Octave now lead to error messages.

Thanks,

Hermann