Dear Ansley and Billy,
Thank you for your help! Here I summarize what I've found so far.
1. "ppl xfor,(i5,''@im09'')" works with "set text/font=HERSHEY", which is Ferret's native font.
2. With system fonts (but I have tried just one), the number and the text overlap! So, something like
ppl xfor,(i5,''___°'')
may kind of work if you replace the underscore "_" characters with non-visible white space characters from unicode (untested). I've just tested this idea with the underscores.
3. "cancel mode long_label" does disable "E" and "W" but also disables degrees symbols:
yes? set data coads_climatology
yes? cancel mode long_tabel
yes? fill/L=1 SST
In this case, Ferret stops its special treatment of longitude axes. So, you still need to add degrees symbols by yourself.
Cheers,
Ryo
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Out of curiosity, I checked what gfortran does with unicode characters. These days, unicode characters are nothing to worry about in modern languages. So, this program
program try
write(*,"(I5, '°')") 3
end program try
produced 3° on my screen.