placement of figured bass

[Alan Jones]

I've been trying to use a combination of figured bass and lyrics to
provide romain numeral annotation to music.  In the following example,
the figured bass is placed far below and to the right of the corresponding
lyric:

\version "2.10.17"
\relative c' {
  c d e f
}
\addlyrics { 
  I V IV ii
}
\new FiguredBass \figuremode {
  <6> <6 4> <6> <6 4>
}

Adding the following lines after "\figuremode {" shows that the horizontal
spacing is actually increasing:
  \override BassFigure #'font-size = #-2
  \override BassFigure #'extra-offset = #'(-10 . 10)

I have also tried entering separate "\new FiguredBass {...}" as elements 
in the list of lyrics.  This resulted in increasing vertical offsets.  The
increasing offsets eventually cause problems with the page layout.
Any suggestions?
Alan