Move chord name down onto staff?

[Mark Probert]

Hi, all.

I'm not sure how to move the chord names down so they are centered on 
the staff. Essentially I want the changes on an empty stave but I'm 
unclear on how to do that.

TIA .. mark.

--
\version "2.20"

harmony =  \chordmode { a1:m s1 f1:m7 s1 }
tune = { s1 s1 s1 s1 }

\score {
  <<
    \new ChordNames \harmony
    \new Staff      \tune
  >>
}
--