Markups crossing staff lines

[James Worlton]

Hello,

I've been trying to get certain markups to overlay staff lines in a keyboard score and have made some progress using \tweak #'extra-offset. However, it appears that the space generated by the markup remains between the staves after the object has moved. The following code demonstrates this:

\version "2.17.14"

bracketpath = #'((moveto -0.75 -2.5)
                 (lineto -0.75 4.2)
                 (lineto 0.5 4.2))

bracket = ^\markup {
  \path #0.15 #bracketpath
}

notes = \relative c' {
  c8 d e f g2
}

\score { %space nice & tight between staves
  \new PianoStaff <<
    \new Staff { \notes }
    \new Staff { r4 a'2. }
  >>
}

\score { %extra space between staves
  \new PianoStaff <<
    \new Staff { \notes }
    \new Staff { r4 a'2.^\tweak #'extra-offset #'(-1 . -6) \bracket }
  >>
}

Is there a better way to create markups that overlay staff lines than what I'm doing? Or is there a way to reduce the unnecessary space that shows up in my second example?

Thanks,
James Worlton