Intervalic Chord Names

[Tom Swan]

Hello. I need some help figuring out how to change chord names (C, F, G) into intervals such as (I, IV, V). The short example below displays a timing diagram for a melodic line, but I would also like to display the chords C7, F7 as generic intervals I7 IV7. Manually setting the text would okay -- I'm not looking for some kind of automatic translation, although that would be neat. I know I can do this with markup, but I do not want to add markup statements to noteValues! Do I need to set chordRootNamer somehow? Or is there another solution? Thanks for any advice.

\version "2.18.2"

globalDefs = {
\clef "treble_8"
\key c \major
\time 4/4
}

noteValues = {
a4 a4 a8[ a8] a4 |
a4 a4 a2 |
a4 a8[ a8] a4 a4 |
a2 a8[ a8] a4 \bar "|."
}

chordValues = \chordmode {
% \set chordRootNamer = ???
c1:7 f1:7 c1:7 c1:7
}

\score { <<
\new ChordNames {
\chordValues
}
\new RhythmicStaff = "Timing" <<
\globalDefs
\new Voice = "Rhythm" {
\globalDefs
\omit RhythmicStaff.Fingering
\improvisationOn
\noteValues
} % Rhythm Voice
>>
>>
}

http://www.tomswan.com