Harry Ohlsen writes,

# >Have you looked at the Tk widget demos on RAA? It's been a while since 
I 
# >looked at them, but I wouldn't be surprised if pieces of what you 
wanted 
# >were already there.
# 
# Yes.  The problem is, of course, that apart from grep-ing for things
# it's hard to find code that gives a clue to how to handle a specific
# problem.

OK. (I presume you have run the big widget demo with the various canvas 
sub-demos, and used the show-code buttons, which is a little easier than 
grepping.

# Sorry, I realise I'm trying to do things after a week that maybe
# people normally would know how to do if they'd taken some time to
# learn Tk.

Well, I have only learned (and mostly forgotten by now) the basics, plus I 
don't have time to look into this, unfortunately.

If no one else pops up with an answer, you might want to resort to 
comp.lang.tcl and translate the answer to Ruby.

Conrad Schneiker
(This note is unofficial and subject to improvement without notice.)