Combining diacriticals

[Nicholas Bastin]

I'm unclear as to whether this is a font problem, or an OmniGraffle  
problem.

I'm trying to display the character xbar, which is unicode 0305 0078  
(a bar diacritic with an x).  However, Omnigraffle just makes these  
two different characters.  0305 is definitely defined by the Unicode  
Standard as a "Combining Diacritical Mark" and so it should not  
render as its' own character.  All fonts I've tried that support the  
0300-036F code points have functioned in this way.  Is there any non- 
hackish way to display these characters?

--
Nick