Move markup relative to parent from inside define-markup-command?

[Nick Payne]

I'm using define-markup-command to simplify the indication of barring 
for a single chord, but what I have at the moment defaults to being 
centred on the note, and the vertical line indicating how many strings 
to cover is overlaid by the notehead unless the markup position is 
tweaked. Is it possible, if I have an additional numeric argument for 
the amount of shift needed, to apply that shift from inside the 
define-markup-command rather than having to use an external tweak? This 
is what I have at the moment:

\version "2.15.32"

#(define-markup-command (mbarre layout props fretnum vdrop) (markup? 
number?)
    (interpret-markup layout props
    #{ \markup \small \bold { \concat { \postscript #(string-append 
"0.1 setlinewidth 0.7 -0.5 moveto 0 -"
        (number->string vdrop) " rlineto 0.5 0 rlineto stroke") 
$fretnum } } #}
    )
)

\relative c'' {
<e c>4^\markup { \mbarre "CV" #2.3 } <e c>-\tweak #'extra-offset #'(-1.5 
. 0)^\markup { \mbarre "CV" #2.3 }
}

Nick

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