Staff: vertical spacing, reduce chord name distance

[Thomas Scharkowski]

IIRC in previous versions I could reduce the distance between a staff 
and chord symbols by this command:

\set Staff.minimumVerticalExtent = #'(-4 . 1)

In 2.8.1 this is:
\override Staff.VerticalAxisGroup #'minimum-Y-extent = #'(-4 . 1)

or did I get something wrong?
Reducing the space this way dos not work here, increasing does work.
Can anyone point me in the right direction?

Thank you
Thomas
Windows XP

code:
--
\version "2.7.40"

#(set-default-paper-size "a6" 'landscape)
#(ly:set-option 'point-and-click #f)
#(set-global-staff-size 18)

Melody =        \context Voice = "Melody" {
                \relative c' {c4 d e r }
                }
        
ChordSymbols =  \context ChordNames 
                \chordmode  {c1}

GuitarStaff =   \context Staff
                <<
                %\override Staff.VerticalAxisGroup #'minimum-Y-extent = #'(-4 . 
12) 
% works!
                \override Staff.VerticalAxisGroup #'minimum-Y-extent = #'(-4 . 
1) % 
does not work!
                \Melody
                >>
                
Text =          \lyricsto "Melody"  \new Lyrics  { Wet -- ter -- frosch, }      
                
\score 
{
        <<
        \ChordSymbols
        \GuitarStaff
        \Text
        >>
        
        \layout {}
}
\paper {}