Run graphical program from crontab

[Mattia]

Hi all,
I'm trying to solve a problem, but I can't guess the solution: I have
written a small program, with a graphical interface, and I would like to
launch it from the crontab, so that it would appear on my desktop.
I've tried to put it into the system-wide crontab (/etc/crontab) or in
the user-specific crontab (crontab -e) but with no success: the program
is executed, but it doesn't appear in my graphical interface.

I'v tried also to launch the program from a bash script, which is then 
executed from thr crontab. This is the script:

#!/bin/bash
export DISPLAY=localhost:0.0

/home/esperimenti/tcltk/test3.tcl >> /root/output 2>> /root/errori

Thanks a lot in advance.

Mattia