Quidelines for Site visitor counter

Hello, I would like to add a visitor counter to my site. I know that there are plenty of free and nice applications/scripts for this avalable but I would like to write my own. I'm at the moment thinking either to make graphical or text based user interface for this statistics counter. So I'm asking your advice on how to draw those nice graphs with PHP? Any online manuals or books that can help since I've never done anything like this before. The server supports drawing graphical stats with PHP. Here's a link that shows roughly what I'm looking for: http://bbclone.de/demo/show_time.php?lng=en Thanks -Will