making a dashed line from one staff to another

[DJF]

I’m trying to draw a dashed line between notes in two staves. Ideally (using my example below), the line needs to start in the middle of the left-hand staff immediately above the first grace note and be long enough to end directly below the E-flat of the right hand. I can only manage one or the other … If I make the line longer (increasing the 6.9 #), it moves the staves apart even more, without changing where the line begins or ends in relation to the pitches. If I adjust the extra-offset, I can make the line start where I want, but that results in too much white space below the E-flat. Is there a way to get rid of that white space, or increase the length of the line without moving the staves apart, or is there a better way to approach this? 

The first picture uses the code below (with the line not starting low enough, but ending at the right spot). The second shows the white space if the Y of the extra-offset is set to -2.3 (the line starting where I want, but not ending where it should).

I’ll be grateful for any advice.

—

Dan

\version "2.19.39"

rightOne = \relative c' {
 f16 aes des ees ees,16 aes c ees
}

leftOne = \relative c' {
  \tuplet 3/2 { c8 bes aes' } 
  \once \override TextScript.extra-offset = #'( 1.0 . 0.0 )
   \acciaccatura { bes,16^\markup { 
         \override #'(thickness . 0.7)
         \override #'(off . 0.3)
         \draw-dashed-line #'(1.9 . 6.9) }
       c } bes4  
}

\score {
  \new PianoStaff 
  <<
    \new Staff = "right"
      { \rightOne }
      \new Staff = "left"
      { \leftOne }
  >>
}