Draw a line on the X reference point of a grob

[Paolo Prete]

Hello. IIUC, the reference point for the X-offset property (or any other x-shifting property that preserves the avoid-collisions algo) is on the X- parent grob of the grob I want to modify.

Is there a way to draw a line on this grob, so that I can visually well see the distance that I'm going to modify?

I tried to use the ly:grob-parent function, but there's obviously some error in the below code, as it doesn't show anything:

%%%%%%%%%%%%%%%

{

\tweak DynamicText.after-line-breaking
#(lambda (grob)
(let* ((grob-x-parent (ly:grob-parent grob 0)))
    (ly:grob-set-property! grob-x-parent 'stencil
#{ \markup { \override #'(thickness . 3) \draw-line #'(0 . 3) } #})))

c'\mf
 
}

thanks,

P