Reducing the spacing between ChordNames systems

[Richard Shann]

With Chord Charts there are only chord symbols - no melody as in a lead
sheet. I have been trying to control the system-to-system spacing of
these lines of chord symbols without success.
Reading 
http://lilypond.org/doc/v2.18/Documentation/notation/flexible-vertical-spacing-within-systems
I thought VerticalAxisGroup.default-nonstaff-nonstaff-spacing was the
thing I needed to control, but it seems not. Can someone say what is
controlling the spacing?
Here is an example- a series of lines of root chords on C, each line
spaced much too far apart for this application, despite setting all the
VerticalAxisGroup defaults I could find.

\version "2.18.0"

MvmntIIVoiceI = {
           c1 c c c
            \break
            c1 c c c
             \break
            c1 c c c
            \break
            c1 c c c
           }
           
\score {
    <<  \new ChordNames  \MvmntIIVoiceI >>
\layout {

        \context {
            \Score
            \override
VerticalAxisGroup.default-nonstaff-nonstaff-spacing =
#'((basic-distance . 0)
               (minimum-distance . 0)
               (padding . 0)
               (stretchability . 0))
               \override VerticalAxisGroup.default-staff-staff-spacing =
#'((basic-distance . 0)
               (minimum-distance . 0)
               (padding . 0)
               (stretchability . 0))
            \override
VerticalAxisGroup.default-nonstaff-relatedstaff-spacing =
#'((basic-distance . 0)
               (minimum-distance . 0)
               (padding . 0)
               (stretchability . 0))

            \override
VerticalAxisGroup.default-nonstaff-unrelatedstaff-spacing =
#'((basic-distance . 0)
               (minimum-distance . 0)
               (padding . 0)
               (stretchability . 0))
          }
    }
}

I'm thinking it must be that VerticalAxisGroup is a between-system
thing, what will do the system-to-system thing when there are no
ordinary staffs?

Richard