myfunction = #(define-music-function (parser location event-chord numx)
                                                    (ly:music? number?)
#{ 
 \once \override TextScript #'outside-staff-priority = ##f
 \once \override TextScript  #'X-offset = $numx
 \once \override TextScript #'Y-offset = 
   #(lambda (grob)
     (let* ((paper-col (ly:grob-parent grob X))
            (elts (ly:grob-object paper-col 'elements))
            (rhythmic-head grob))
       (for-each
          (lambda (idx)
            (let ((elt (ly:grob-array-ref elts idx)))
              (if (grob::has-interface elt 'rhythmic-grob-interface)
                  (set! rhythmic-head elt))))
          (reverse (iota (ly:grob-array-length elts))))  
       (- (/ (ly:grob-staff-position rhythmic-head) 2) ; well, don't know why
                                                       ; we have to divide by 2?
          (interval-center
              (ly:grob-robust-relative-extent grob grob Y)))))
 $event-chord             
#})              


\score{
\relative c'{
  \myfunction c1-\markup{\sans "<-"} #3
  \myfunction g'1-\markup{\sans "<-"} #3
  \myfunction c1-\markup{\sans "<-"} #3
  }
}