Vertically aligning fret diagrams

[Nick Payne]

If I have a succession of fret diagrams above the chords on a stave, how 
do I align the fret diagrams vertically. In the example below, the chord 
names at the top of all three fret diagrams are aligned vertically, but 
the fret diagrams for A and C have more space below the chord name and 
sit lower than that for G.

\version "2.15.29"
\language "english"

<<
    \context ChordNames {
        \chordmode {
            \override ChordNames.ChordName #'font-size = #4
            \override ChordNames.ChordName #'#'extra-offset = #'(0.4 . 0)
            a1 c g
        }
    }
    \context Staff {
        \clef "treble_8"
        \set fingeringOrientations = #'(left)
        \override Fingering #'padding = #0.3
        \override Staff.AccidentalPlacement #'left-padding = #0
        \override TextScript #'size = #'1.5
        \override TextScript #'(fret-diagram-details finger-code) = 
#'in-dot
        \override TextScript #'extra-spacing-width = #'(-1 . 1)
<a, e-\tweak #'extra-offset #'(-1.4 . 0)-1 a-\tweak #'extra-offset 
#'(-1.4 . 0)-2 cs'-3 e'>^\markup {
            \fret-diagram #"6-x;5-o;4-2-1;3-2-2;2-2-3;1-o;"
        }
<c-3 e-2 g c'-1 e'>1^\markup {
            \fret-diagram #"6-x;5-3-3;4-2-2;3-o;2-1-1;1-o;"
        }
<g,-2 b,-1 d g b g'-3>1^\markup {
            \fret-diagram #"6-3-2;5-2-1;4-o;3-o;2--o;1-3-3;"
        }
    }
>>

chords.png
Description: PNG image