How to sumarize longotudinal data by patients and plot it?

Question

[Lila M]

Hi all, I'm trying to summarize some longitudinal data that looks like this:

ID Time_point   treatment
20  4            Treated
20  7            Treated
20  9            Treated
20  12           Treated
20  21           Treated
 28 4            No Treated
28  7            No Treated
28  12           No Treated
28  14           No Treated
28  18           No Treated
28  21           No Treated
29  4            Treated
29  7            Treated
29  12           Treated

The principal characteristic of the data is that not all the IDs has the same time points. I would like to plot the data (by patients) using some bar plot in R, any idea about how to do it? Thanks!

[Ram]

Can you describe what this bar plot would depict, and what its axes would be?