You need to pass your CGI a parameter to tell it which graph you want. It obviously can't know which part of the page it's being called for (in fact, it doesn't even know there is a link to it on a page). After all, if you type the CGI's URL in your browser address bar, how would it decide it what to do? Append something like ?type=bar to the URL and read up on CGI's param() function.

Makeshifts last the longest.