Spacing between staves

[Richard Opheim]

I used to be able to control the space between staves using 

\override VerticalAxisGroup #'minimum-Y-extent 

but recently, it seems to have lost functionality. For example, in the snippet 
below, no matter what numerical values I insert into the parentheses, the 
ossia staff doesn't change its (default?) position in relation to the main 
staff.

\version "2.14.0"
\layout {\context {\Score }}

<<\new RhythmicStaff = "main"
\with {\override VerticalAxisGroup #'minimum-Y-extent = #'(-1 . 1) }
<<\time 6/8 
{ b4. r4. b4. r4.}

%ossia stave
\new RhythmicStaff \with {alignAboveContext = #"main" fontSize = #-3}
{b4 b8 r8 b4 b8 b4 b4 b8}>>>>