Bug#1026986:

[Παύλος Γκέσος <gessos.paul@gmail.com>]

Great, but not 100% fixed.

The following functionality is not fixed:

WHAT MUST HAPPEN:
Type KEY                   ; (in Greek is the CHAR "'") - This is a
"dead" key and nothing happens
Type KEY COMBINATION Shift-; (in Greek is the CHAR """) - This is a
"dead" key and nothing happens
Type KEY                   y (in Greek is the CHAR "υ"). The CHAR "ΰ"
must appear in tty (you have it with Alt+0944 key combination)

What HAPPENS AFTER LAST UPDATE:
Type KEY                   ; (in Greek is the CHAR "'") - This is a
"dead" key and nothing happens - CORRECT
Type KEY COMBINATION Shift-; (in Greek is the CHAR """) - The CHAR "'"
appears in tty - WRONG! (This is a "dead" key and nothing must happen)
Type KEY                   y (in Greek is the CHAR "υ"). Two CHARs
"'ϋ" appear in tty - WRONG! (This is not 'ϋ but ΰ)
So, there are 2 consecutive dead keys here.

This functionality exists only for 2 characters:
ι => ΐ     (keys ;   Shift-;   i       OR      Shift-;   ;   i)
υ => ΰ   (keys ;   Shift-;   y       OR      Shift-;   ;   y)
More info here under the header "Greek" (plain):
http://kbdlayout.info/features/deadkeys

The MS Windows implementation method (just for reference - it breaks
the SAME keyboard layout between tty and X):
Instead of keys ; Shift-; y to get character "ΰ" (linux way)
they use Shift-w y to get character "ΰ" (Windows way)
(This produces 2 characters "Συ" in Linux).