horizontal/vertical text spanner alignment

[Patrick Schmidt]

Dear all,

how can I align a text spanner vertically and horizontally to get something 
like this ➂---➁? Here is a tiny example:


\version "2.13.3"

scale = \relative c'' {
        %\once \override TextSpanner #'(bound-details left Y) = #5
        %\once \override TextSpanner #'(bound-details right Y) = #5
        <g\3_0>\startTextSpan <a_2> <b_4>\stopTextSpan <c\2_1>
}

\score {
        \new Staff \scale
        \layout {}
}

I fiddled with \once \override TextSpanner #'(bound-details left/right Y) = 
#[±0-9] but to no avail.

Any hint is greatly appreciated.

Thanks for your help!

patrick
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