(from http://serverfault.com/questions/53080/how-to-enable-ssh-x11-forwarding-through-additional-server)
Local linux box (llb) can ssh only to cube.example.org on port 212, from cube can ssh to ninja;
Step 1; forward local port to port 22 on ninja
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user@workbench:~$ ssh -L 2222:ninja:22 -p 212 mythtv@cube.example.org
Step 2; in another terminal, connect to local port;
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user@workbench:~$ ssh -X -p 2222 localhost
..
xclock