Alternately, don't produce the chart at all - have that done in another runmode (using CGI::Application terms) or another CGI script or whatever. And then set the cache time to 0 in your headers before printing it out. This prevents the race condition mentioned above, eliminates the client's tendency to cache it, and probably allows you to use the server CPU time more effectively (since these will be two processes rather than one), and also start sending back the HTML sooner (since producing the chart won't completely stop producing the HTML).

CGI scripts can send back images just fine - you just have to manipulate the header appropriately.