fret diagrams with marked root and roman numbers

[BB]

 \markup
    \override #'(fret-diagram-details . (
           (number-type . arabic)
           (finger-code . in-dot))) {
      \fret-diagram #"s:3;f:1;6-3-2;5-2-1;5-5-4;4-4-3;"
      % percent sign is for comments
      % s:3 tells lilypond to make diagram 3x normal size
      % f:1 is to show finger #'s in circle on fret
      % 6-3-2 is sixth string - third fret - 2nd finger
      \fret-diagram #"s:3;f:1;5-4-2;4-3-1;4-6-4;3-5-3;"
     }

I try to put root, I, III V etc into the circles instead of the arabic numbers. I screened the manual - no hit. I did a google search - no hit.
I found no documentation for (number-type . arabic) either - might propably switch to characters?

(number-type . ascii) is working but does not display ascii characters?

Is  there an easy way?

Xmas greetings