coloring individual pixels

Can someone help me out with an example?

I want to color all the individual pixels in a pixmap according to
some mathematical function, and I'm just confused as heck.

Here's some snippets of non-working code which will hopefully give
somewhat of an idea what I'm trying to do ... 

.... once I've set up my pixmap (which I don't have right! Ack!)

   $gc = new Gtk::Gdk::GC ( $widget->window );
   for ($x=0; $x<$width; $x++) {
      for ($y=0; $y<$height; $y++) {
         $gc->set_foreground($widget->window->get_colormap->color_alloc
                             (f_color($x, $y)));
         $pixmap->draw_point($widget->window, $gc, $x, $y);
      }
   }

....

sub f_color {
   my ($x, $y) = @_;
   # this will eventually do some clever math thing, but 
   # returning just one color is fine for now.
   return { red => 65000, green => 32500, blue => 0 };
}

I'm trying to go from the example at http://www.gtkperl.org/sample-chart.html,
but that doesn't tell me how to work with individual pixels.

Everything I'm trying is failing.  Any help would be most appreciated.

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