Modifying vectors for plotting...

Hi I have the following vectors

x(:,1) = 1 1 2 2 3 4 4 5 6 6 … 120 etc; % There is no pattern. These are the session #s.
y(:,1) = 10 20 20 30 40 40 40 50 …etc..% These are the levels.
a(:,1) = 1 2 3 4 5 6 7..120% % Session #s as in x
b(:,1) = 12345 12345 12345 12346 12346 etc %dates associated with the session.

a and b go together. i.e. a has unique sessions from 1 to 120. b has the associated date. Note multiple sessions might have been completed on the same date.

x and y go together. I want to plot session vs levels (x vs y) BUT need to pay attention to dates. i.e. if multiple sessions (1, 2 ,3) were run on the same date (b), then they have to be stacked together..

THanks!

S