horizontal line separator in column markup (as in fraction or polychord)

[Jean-Alexis Montignies]

Hi,

I'm trying to write a polychord markup that looks like a mathematical fraction.

May be I've overlooked something, but I didn't found a way to draw a horizontal 
line between to text lines of a column.
I'm using:

\markup {\override #`(direction . ,UP) \dir-column 
      { "C"
        \draw-line #'(3 . 0)
        "Db" }
}

But then I have too many vertical space between Db and the separator and I have 
to hardcode the length of the line.

Any suggestion?

Thanks