Forgive the blind translation attempt, but wouldn't something like this work?
hungerpct = 1:100;
bph = (1:100).'.*[2 3] + [0 5] + 5*rand(100,2);
hp = plot(hungerpct,bph);
legend(hp,{'Diddy Kong','Donkey Kong'})
xlabel('Πείνα (%)')
ylabel('Ποσοστό κατανάλωσης μπανάνας (μπανάνες/ώρα)')
Unicode characters should generally be supported, so this should work. If on the other hand, you're trying to feed it something like "banánas" and have the figure automatically do the character translation to "μπανάνας", that might be a different story.
