Lilypond: adding arrows between cells of a chord grid - lilypond

Question

I filled a chord grid in LilyPond which you can see the output here. I want to add some arrows to write a syncope.
I thought about adding slurs:
grille = \chordmode {
\bar "[|:"
d1(
g1)
...
But it doesn't modify anything.
Here is the code:
% LilyBin
\version "2.19.55"
#(set-global-staff-size 18)
% si cette valeur change modifier la valeur de rlineto ci-dessous
\paper {
page-breaking = #ly:page-turn-breaking
}
\header {
tagline = ##f
composer = "The Cure"
title = "Friday I'm in Love"
}
\paper {
#(set-paper-size "a4")
page-count = #1 % forçage du nombre de pages
page-breaking = #ly:page-turn-breaking
}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% La fonction pour diviser une case en 2 %%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
"\\/" =
#(define-music-function
(parser location mus1 mus2) ( ly:music? ly:music? )
#{
\stopStaff
\once \override Staff.StaffSymbol.stencil =
$(lambda (grob)
(let* ((staff (ly:staff-symbol::print grob))
(X-ext (ly:stencil-extent staff X))
(Y-ext (ly:stencil-extent staff Y)))
(ly:grob-set-property! grob 'layer -10)
(ly:stencil-add
(make-line-stencil
(ly:staff-symbol-line-thickness grob)
(car X-ext) (car Y-ext) (cdr X-ext) (cdr Y-ext))
staff)))
\startStaff
\once \override ChordName.extra-offset = #'(1 . .5)
$mus1
\once \override ChordName.extra-offset = #'(3 . -3)
$mus2
\stopStaff
\startStaff
#})
%%%%%%%%%%%%%%%%%
grille = \chordmode {
\bar "[|:"
d1
g1
d1
a1
\break
b1:m
g1
d1
a1
\bar ":|]"
\break
\bar "[|:"
d1
g1
d1
a1
\break
b1:m
g1
d1
a1
\bar ":|]"
\break
g1
a1
b1:m
g1
\break
d1
a1
\stopStaff
\bar "" s \bar "" s
\break
\startStaff
d1
g1
d1
a1
\break
b1:m
g1
d1
a1
\break
d1
g1
d1
a1
\break
b1:m
g1
d1
a1
\break
d1
g1
d1
a1
\break
b1:m
g1
d1
a1
\pageBreak
\break
g1
a1
b1:m
g1
\break
d1
a1
\stopStaff
\bar "" s \bar "" s
\break
\startStaff
\set Score.repeatCommands = #'((volta "4 X"))
\bar "[|:"
b1:m
c:9
d1
a1
\set Score.repeatCommands = #'((volta "#f"))
\bar ":|]"
}
marques = \relative c' {
s1 ^\markup \bold \box \fontsize #7 Intro s1*7
s1 ^\markup \bold \box \fontsize #7 VerseI s1*7
s1 ^\markup \bold \box \fontsize #7 Chorus s1*7
s1 ^\markup \bold \box \fontsize #7 VerseII s1*7
s1 ^\markup \bold \box \fontsize #7 Solo s1*7
s1 ^\markup \bold \box \fontsize #7 VerseIII s1*7
s1 ^\markup \bold \box \fontsize #7 ChorusII s1*7
s1 ^\markup \bold \box \fontsize #7 Bridge s1*7
}
\score{
\layout {
indent = 0 % pas de décalage même à la première ligne
\context {
\Score
\omit Clef % Cacher la clef
\omit TimeSignature % cacher la métrique
\omit BarNumber % cacher les numéros de mesures
\override SpacingSpanner.strict-note-spacing = ##t % permet d’avoir toutes les mesures de même longueur
proportionalNotationDuration = #(ly:make-moment 1/16) % durée de référence pour espacer la musique
}
}
<<
\new Staff \with { % la portée qui recevra les repères
\remove "Staff_symbol_engraver" % cacher la portée
}
\marques % la variable contenant les repères
\new ChordNames \with { % la portée qui recevra la grille harmonique
\override ChordName.extra-offset = #'(10 . -1 ) % déplacer les noms des accords
\override BarLine.bar-extent = #'(-5 . 5) % définir la taille des barres de mesures
\consists "Bar_engraver" % afficher les barres de mesures
\override StaffSymbol.line-positions = #'( -10 10 ) % definir une portée de 2 lignnes
\consists "Staff_symbol_engraver" % afficher la portée
\consists "Percent_repeat_engraver" % afficher le signe "%" pour les mesures répétées
}
\grille % la variable contenant la grille harmonique proprement dite
>>
}

Answer 1

I'm new to Lilypond myself, but I was able to get the arrow by creating a custom shape in the markup of a measure /mark.
See this example (based on your original output): http://lilybin.com/k7kykq/142
Please forgive the fact that I hard-coded the shape. I don't yet know of an easier way of drawing it.
Idea inspired by: http://lsr.di.unimi.it/LSR/Item?id=1050