How do I stop circles from wobbling?

[David Kastrup]

The following is wobbly, likely because of different vertical extends of
the strings.  But the vertical extent should fit in the circles
necessitated by the horizontal extent.

Can anybody tell me what to do to keep the circles from wobbling?

#(define-markup-list-command (buttons layout props str) (string?)
  (map (lambda (str)
        (interpret-markup layout props (markup #:hcenter-in 10 #:circle 
#:hcenter-in 4 str)))
   (string-split str #\-)))

\markup {
  \left-column {
  \line { \hspace #10 \buttons #"D-F-G♯-H-d-f-g♯-h" }
  \line { \hspace #8 \buttons #"C♯-E-G-A♯-c♯-e-g-a♯" }
  \line { \hspace #6 \buttons #"C-D♯-F♯-A-c-d♯-f♯-a" }
  }
}


-- 
David Kastrup