Try using a histogram N = HIST(Y,M) M = the number of bins in this case three. It will equally divide them up.
else you could use a simple case statement where "mild" = 1,2,3 "medium" = 4,5,6,7 "Strong" = 8 9 10 Catagorize them as 1 2 0r 3 according to the score and then try another histogram
Hope that helps