Chord Names set at different heights?

[Walter Hofmeister]

Hello,
    I am engraving a solo guitar arrangement of a traditional folk tune that
has chord symbols above the music staff. The problem is that when the melody
goes higher on the staff, more space is required between the staff and the
chord names. But when the melody is not so high, then you end up with too
much space between the staff and the chord names. I would like to be able to
set the height of the chord names to be a different space for each line of
music if necessary. A setting like padding would do the trick but if there
is one I have missed it in the documentation. I have used  \set
Staff.verticalExtent = #'(-4 . 7) which has worked well. I tried to use
several of these tweaks on different lines of music but always the one which
produces the highest space is the one that seems to prevail. Does any one
have an idea how this could be accomplished? I have included the lilypond
source file here as an example.
    Also there is a bar numbering problem in the second section of the piece
probably caused by the two partial measures. Any suggestions as to how to
fix this? TIA

Walter Hofmeister
P.S. I am using 2.1.28 on Mac OS X.3.3

-----------------File begins here----------------------------------------

\version "2.1.22"
\include "english.ly"
\header {
%dedication = "dedication"
title = "Roslyn Castle"
%subtitle = "Subtitle"
%subsubtitle = "Subsubtitle"
composer = "Traditional Scottish"
%opus = "Opus 0"
%piece = "Piece I"
%instrument = "Instrument"
%arranger = "Arranger"
%poet = "Poet"
%texttranslator = "Translator"
copyright = "public domain"
%source = "urtext "
enteredby = "Walter Hofmeister"
%maintainerEmail = "your email here"
%texidoc = "The standard header that ought to be in top of a file."
tagline = "Public Domain"
}

harmonies = \chords {
    \override ChordName   #'font-family =#'roman
    \time 4/4
    \partial 8*2
    s8*2
    e2:m g |
    a2:m b:7   |
    e1:m   |
    b1:7   |
%5
    a2:m b:7   |
    e2:m b   |
    e4:m b e:m a:m   |
    e4:m b:7 e2:m   |
    \set chordChanges = ##f    e1:m   |
%10
    a2:m b:7   |
    e1:m   |
    a2:m b:7   |
    a2:m b:7   |
    e2:m b   |
%15
    e4:m b e:m a:m   |
    e4:m b:7 e4:m   |
}

melody = \notes \relative c'' {
    \voiceOne
    \time 4/4
    \key e \minor
    \clef treble
    \repeat volta 2 {
    \partial 8*2
    g8 fs
    <e b>4 b'8 c <b d, g b,>4 a8 b
    <c e, a>8[ b] a[ g] fs4 g8 fs
    <e b>4 e'8 fs <g b, g>4 fs8 e  |            \break
    <ds a fs>8[ e] fs[ ds] <b a ds,>4 a8 b
%5    
    <c a e>8[ b] a[ g] fs4 g8 a
    <b g e b>8[ g] fs[ e] <ds' a fs>4 e8 fs            \break
    <g b, g e b>16 e8. <fs ds a fs>16 ds8. <e b g>8[ b] <a e>[ b]
    %\partial4*3
    <g e>4 fs8 e <e b>4
    }
    \repeat volta 2 {
    %\partial8*2
    e8 b'
    <e b g e b>4 <e b g>8 fs <g b, g e b>4 <fs b, g>8 e            \break
%10
    <b' e, c a>8[ a] g[ a] <fs ds a fs>4 e8 ds
    <e b g e b>4 <e b g>8 fs <g b, g e b>4 <fs b, g>8 e
    <b' e, c a>8[ a] g[ a] <fs ds a fs>4 a,8 b            \break
    <c a e>8[ b] a[ g] fs4 g8 a
    <b g e b>8[ g] fs[ e] <ds' b fs>4 e8 fs
%15
    <g b, g e b>8[ e] <fs ds a fs>[ ds] <e b g>[ b] <a e>[ b]
    \partial4*3
    <g e>4 fs8 e <e b>4   %\bar "|."
    }
}

bassNotes = \notes \relative c' {
    \voiceTwo
    \time 4/4
    \partial8*2
    s8*2        |
    e,2 g        |
    a2 b        |
    e,2 e'    |
    b2 b        |
%5
    a2 b        |
    e,2 b'    |
    e,4 b' e a,        |
    \partial4*3
    b4 b e,
    \partial8*2
    s8*2
    e8 b' e4 e,8 e' ~ e4    |
    a,2 b        |
%10
    e,8 b' e4 e,8 e' ~ e4        |
    a,2 b        |
    a2 b        |
    e,2 b'    |
    e,4 b' e a,        |
    \partial4*3
    b4 b e,        |
}


\score {
\notes 
  <<
    \context ChordNames {
    \override ChordName   #'font-size =#'-1
    \set chordChanges = ##t
    \harmonies
    }
    \context Staff = one <<
        \context Voice = one \melody
    \context Voice = two \bassNotes
    >>
    \set Staff.verticalExtent = #'(-4 . 7)
    \set Staff.minimumVerticalExtent = #'(-6 . 4.5)

  >>

  \paper{ }
  \midi{ }
}