\relative c'' {
  \override Staff.OttavaBracket.stencil = #ly:line-spanner::print
  \override Staff.OttavaBracket.bound-details =
    #`((left . ((Y . 0)
                (attach-dir . ,LEFT)
                (padding . 0)
                (stencil-align-dir-y . ,CENTER)))
       (right . ((Y . 5.0) ; Change the number here
                 (padding . 0)
                 (attach-dir . ,RIGHT)
                 (text . ,(make-draw-dashed-line-markup
                           (cons 0 -1.2))))))
  \override Staff.OttavaBracket.left-bound-info =
     #ly:horizontal-line-spanner::calc-left-bound-info-and-text
  \override Staff.OttavaBracket.right-bound-info =
     #ly:horizontal-line-spanner::calc-right-bound-info
  \ottava 1
  c1
  c'''1
}