Draw a guide line between piano staves in LilyPond

Question

I am trying to reproduce a piano score in LilyPond. It has a straight dash line between a grace note in one staff and a chord in another. I tried using \change Staff, but it either breaks the grace notes, or notes horizontal position, or makes the chord in another staff look small as grace notes.

The picture shows the desired effect.

MWE (without the dash line):

\version "2.20.0"
\language "italiano"

\score {
  \new PianoStaff <<
    \new Staff = "RH" \relative do' {
        \time 3/4
        mi'16 la mi re
        \appoggiatura {
            re32 % need a line from this re down to the second chord in the LH
            mi
        } re8 mi re4
    }
    \new Staff = "LH" \relative do' {
        \time 3/4
        \clef bass
        <fa, la mi'> <fa la re> <<
        {
            re'8 do
        } \\
        {
            <fa, la>4
        }
        >>
    }
  >>
}

Answer 1

This is quite fiddly. I'd do below. There really might well be an easier way to do this. Line created with VoiceFollower which is used for part writing like fugues and the voice changing staves.

Need to create hidden notes which give reference to start x and y, and end x and y of VoiceFollower. Changed position of VoiceFollower and appoggiatura slur to avoid collision.

Separated the main changes into two variables (makes sense anything more than a line, put it as a variable):

• visibileAppoggiaturaSlurAdjust (I was getting confused with the hidden one so made sense call it visible), and
• lineGoesDown (that's obvious what it's doing!)

Changed chords in same voice in LH because it just made sense.

\version "2.20.0"
\language "italiano"

visibileAppoggiaturaSlurAdjust = \shape #'((0.3 . 0.2) (0.15 . 0.15) (0 . 0.15) (0.2 . 0.2)) Slur

lineGoesDown = {
           \change Staff = "RH"
           \override VoiceFollower.style = #'dashed-line
           \override VoiceFollower.bound-details = #'((right (attach-dir . -0.5) (padding . 0)) (left (attach-dir . -1) (padding . 0)))
           \showStaffSwitch
           \hideNotes \stemDown
           \appoggiatura { \once \override Slur.stencil = ##f  re16 } 
           
           \change Staff = "LH" re,4 
           \hideStaffSwitch \unHideNotes \stemUp
}

\score {
 \new PianoStaff <<
   \new Staff = "RH" \relative do' {
       \time 3/4
       mi'16 la mi re
       \appoggiatura {
           \visibileAppoggiaturaSlurAdjust
           re32 % need a line from this re down to the second chord in the LH
           mi
       } re8 mi re4
   }
   \new Staff = "LH" {
       \time 3/4
       \clef bass
       <<
         \relative do'' {
           
           s4 
           \lineGoesDown
           re8 do |
           
         } 
         \\
         \relative do {
           
          <fa la mi'>4 <fa la re>  <fa la> |
          
         }
       >>
   }
 >>
}

Produces:

---

This second one produces desired output for a repeat of the second bar where there was a bug of the voicefollower being on when it should be off but would later go back on again!

\version "2.20.0"
\language "italiano"

visibileAppoggiaturaSlurAdjust = \shape #'((0.3 . 0.2) (0.15 . 0.15) (0 . 0.15) (0.2 . 0.2)) Slur

lineGoesDown = {
           \hideStaffSwitch
           \change Staff = "RH"
           
           \override VoiceFollower.style = #'dashed-line
           \override VoiceFollower.bound-details = #'((right (attach-dir . -0.5) (padding . 0)) (left (attach-dir . -1) (padding . 0)))
           \showStaffSwitch
           \hideNotes \stemDown
           \appoggiatura { \once \override Slur.stencil = ##f  re16 } 
           
           \change Staff = "LH" re,4 
           \unHideNotes \stemUp
           \once \omit VoiceFollower
          
}

\score {
 \new PianoStaff <<
   \new Staff = "RH" \relative do' {
       \time 3/4
       \repeat unfold 2 { mi'16 la mi re
       \appoggiatura {
           \visibileAppoggiaturaSlurAdjust
           re32 % need a line from this re down to the second chord in the LH
           mi
       } re8 mi re4
       }
   }
   \new Staff = "LH" {
       \time 3/4
       \clef bass
       \repeat unfold 2 {
       <<
         \relative do'' {
           
           s4 
           \lineGoesDown
            re8 do | 
           \change Staff = "RH"
           \hideStaffSwitch
          
         } 
         \\
         \relative do {
           
          <fa la mi'>4 <fa la re>  <fa la> |
          
         }
       >>
      
       }
   }
 >>
}