Make a graph where I will have two lines, one for average, one for certain client

Hi,

I'm trying to make a line graph, as the title says. There should be two lines, one that will always be there, with the average (I have this one already), but now I want to include another one that shows the trajectory of a certain clients. Is there a way to make a drop down list where I can select which client I want to compare to the average?

To clarify, since that was kind of confusing: How do I make a drop down list where I can select clients, and see their increase in scores, and have the ability to just pick another client and then see theirs.

Thanks a lot.