Centering chord diagram at top of page

[address@hidden]

Good evening,

I'm working on a lead sheet and I would like to show chord diagrams at 

the top of the page, as opposed to showing them over the staff for every 

chord. Here's a simplified version of my score...

\version "2.19.11"

\include "predefined-guitar-fretboards.ly"

global = {

  \time 4/4

  \override Staff.TimeSignature.style = #'()

  \key e \major

  \tempo 4=120

}

% Insert blank line

\markup {

  \fill-line {

    \center-column  { 

      " "

    }

  }

}

% Define fretboard diagrams

mychorddiagrams = \chordmode {

  b1 a

}

<<    

  \context ChordNames {

    \mychorddiagrams

  }  

  \context FretBoards {

    \override FretBoards.FretBoard.size = #'1.2

    \mychorddiagrams

  }

>>

% Define chords

chordNames = \chordmode {

  \override ChordName.font-size = #2 

  \global

  % Section A

  b1*2 a1*2

}

% Define melody

melody = \relative c'' {

  \global

  % Start of section A

  \mark \markup{ \tiny \box A } 

  % bars 1-4

  r2 b8 cis dis fis8~ | fis4. fis8~fis8 e8 dis4 | e8 e dis b~b2 | a8 a 

gis a~a gis e4 | \break

}

\score {

  <<

    \new ChordNames \chordNames

    \new Staff { \melody }

  >>

  \layout { indent = 0.0\cm }

  \midi { }

}

This is working fine except for one thing. I would like to center the 

chord diagrams horizontally on the page. I tried with the markup syntax 

that works for text but wasn't successful. Is this possible?

While we're at it, is there a more elegant way to leave a bit of extra 

space above or below the chord diagrams? I can insert a blank line but 

there has to be a better way.

Thanks for your help.