fret-diagram layout question

[Jacques Menu]

Hello folks,

This snippet is adapted from the LPNR, with whole notes replaced by quarter 
notes:

\version "2.19.83"

<<
  \new ChordNames {
      \set chordChanges = ##t
      \chordmode {
      c4 d:m
    }
  }
  \new Staff {
    \clef "treble_8"
    <c e g c' e'>4^\markup {
      \fret-diagram #"6-x;5-3;4-2;3-o;2-1;1-o;"
    }
    <d a d' f'>4^\markup {
      \fret-diagram #"6-x;5-x;4-o;3-2;2-3;1-1;"
    }
  }
>>

\layout {
  \context {
    \Score
   % \override SpacingSpanner.base-shortest-duration = #(ly:make-moment 1/64)
  }
}



How can this be displayed with the two fret diagrams on one and the same 
horizontal line?
Uncommenting the SpacingSpanner.base-shortest-duration override is a bit 
artificial, maybe there’s a better way?

Thanks for your help!

JM