chords grid

[douwen elo]

hi everyone

i now use the following code for having some grids

i guesse that i can transpose them easily with the instruction \transpose

so, that is much more interessant as typing theses chords in an usual text 
editor

for the lines i will superpose it into my text/vectorial editor

now my question is :

how to put two chords in a single case ?

it is very frequent for improvisation ot have somethign like that :

C  |  D/F7  |  G  |  Am

the "D/F7" indicatves that half the time of a case it is D and the other half 
is F7

in Lilypond i can indicate d:/f that means chord D with bass note F, but that 
could be convenient for having D for half and F for other half of the case
now how could i have D/F7 ? because Lily will not accept d:/f:7

Sacha


--- my code (=Xavier's code that i have just lighted) -----


accords = \chordmode {
c:     f:/g    f:m7/c  cis:m7  b:       b:      b:      b:      \break
a:      b       a       a       b:      b:      b:      b:      \break
a:      a       e:m7+   b:9     b:      b:      b:      b:      \break
d:maj   r       a:      b:      b:      b:      b:      b:      \break
a:      d:7     r       aes:m6  b:      b:      b:      b:      \break
bes:7+  c:6     cis:m   d:m9    b:      b:      b:      b:      \break
}
\version "2.15.19"

\paper {
  #(set-paper-size "a4" 'landscape)
  indent = 0
  system-system-spacing = #'((padding . 0)
                             (basic-distance . 0)
                             (minimum-distance . 0)
                             (stretchability . 0))
        left-margin = 0
        right-margin = 0
        top-margin = 0
        bottom-margin = 0
        check-consistency
}



\layout {
  \context {
    \Score
    \accepts "LineUp"
    \accepts "LineDown"
    \remove "Bar_number_engraver"
    proportionalNotationDuration = #(ly:make-moment 1 8)
   % \override SpacingSpanner #'strict-note-spacing = ##t
  }
  \context {
    \ChordNames
    % french chords
    chordRootNamer = #(chord-name->italian-markup #t)
    chordPrefixSpacer = #0.4
   % \consists "Bar_engraver"
    \consists "Double_percent_repeat_engraver"
    \consists "Percent_repeat_engraver"
    \consists "Slash_repeat_engraver"
    \override BarLine #'bar-extent = #'(-3 . 3)
    \override ChordName #'font-name = #'"Apple Chancery"
    \override ChordName #'font-size = #1
    \override ChordName #'Y-extent = #'(-3 . 3)
 %   \override VerticalAxisGroup #'nonstaff-relatedstaff-spacing #'padding = #0
  %  \override VerticalAxisGroup #'nonstaff-nonstaff-spacing #'padding = #0
  }
}

\score {
  <<
 \new ChordNames {
      \accords
    }
  >>

}
\markup {"ambitus :"}
\new Staff {c' d''}