Aligning and centering instrument names

The horizontal alignment of instrument names is tweaked by changing the self-alignment-X property of the InstrumentName grob (usually in the Staff context). The \layout variables indent and short-indent define the space in which the instrument names are aligned before the first and the following systems, respectively.

\version "2.24"

\paper {
  left-margin = 3\cm
}

\new StaffGroup <<
  \new Staff \with {
    \override InstrumentName.self-alignment-X = #LEFT
    instrumentName = \markup \left-column { "Left aligned"
                                            "instrument name" }
    shortInstrumentName = "Left"
  } {
    c''1 \break c''1
  }

  \new Staff \with {
    \override InstrumentName.self-alignment-X = #CENTER
    instrumentName = \markup \center-column { Centered
                                              "instrument name" }
    shortInstrumentName = "Centered"
  } {
    g'1 g'1
  }

  \new Staff \with {
    \override InstrumentName.self-alignment-X = #RIGHT
    instrumentName = \markup \right-column { "Right aligned"
                                             "instrument name" }
    shortInstrumentName = "Right"
  } {
    e'1 e'1
  }
>>

\layout {
  indent = 4\cm
  short-indent = 2\cm
  line-width = 6.5\cm
}