Fine-tuning of markup positions

[Marc Hohl]

Hello,

I have some kind of ornamental "matrix"

a b c
d e f
g h i

I want to display at the end of each \bookpart
(I use different symbols, but this doesn't matter here).

The matrix should be placed centered, and in order to align the
symbols properly,I defined

matrix = \markup {
 \override #'(baseline-skip . 2.2)
 \column { \override #'(line-width . 6.5)
   \fill-line {  a b c }
   \override #'(line-width . 6.5)
   \fill-line { d e f }
   \override #'(line-width . 6.5)
   \fill-line { g h i }
 }
}

and used it via

\markup {
 \fill-line { \matrix }
}

So far, so good; but for the symbols I use, I need some fine-tuning of 
the distances
between b/c and  h/i. In the first case, the symbols should be narrower, 
in the
latter case, I need a small additional space between them.

Inserting a negative \hspace in the first line, for example

 \fill-line { a b \hspace #-1 c }

doesn't have the desired effect.

\translate with negative coordinates doesn't work either.

Is there a different approach I can use?

Thanks in advance

Marc