Yes, you should plot them all to a single large image, otherwise a single client displaying the page will result in 100-500 requests to your webserver. Even all load and performance put aside, it means the page will take a while to display even if you're hitting the server locally over the loopback interface.

All of the graphics generation modules support printing the result to any filehandle you wish, incl. STDOUT, and thus directly to the browser.

I also suggest you take a look at Imager and Imager::Plot instead of GD.

Makeshifts last the longest.