Vertical spacing of figured bass

[Jérôme Plût]

I tried applying the contents of
http://lilypond.org/doc/v2.19/Documentation/notation/flexible-vertical-spacing-within-systems
to move the figured bass a bit closer to the staff:

\version "2.19.54"
\score { { \new StaffGroup <<
      \new Staff { \clef "F" c d e f }
  \new Lyrics \with {
      \override VerticalAxisGroup.nonstaff-relatedstaff-spacing =
      #'((basic-distance . 1) (stretchability . 0))
  } \lyrics { "c"4 "d" "e" "f" }
  \new FiguredBass \with {
      \override VerticalAxisGroup.nonstaff-nonstaff-spacing =
      #'((basic-distance . 1))
  } \figures { <5>4 <5>4 <5> <5> }
>> } \layout { indent = 0 \cm } 

Yet this stupid figured bass does not move, even if I input a
ridiculously large value for 'basic-distance.

What is happening?