Modifying the Ottava spanner slope
It is possible to change the slope of the Ottava spanner.
\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
}