data one;
  input leaf prep1 prep2;
  cards;
1  55 17
2  25 17
3  21 14
4   7 11
5  16 10
6  14  9
7   7  7
8   9  5
9   5  3
10  4  2
11  4  1
12  1  0
;

data one; set one;
  diff=prep1-prep2;
run;

proc univariate plot;
  var diff;
run;