We use an RTG solution for displaying our graphs. I'm a little new to WHMCS, so I'm not sure what kind of module customization we are able to do, but I will see if I can convert my in-house solution over to WHMCS and post it here.
Pretty much what I have setup is a dedicated monitoring server, which has RTG installed on it. I use the RTG cgi that comes with the installation to generate the graphs. I have a PHP file that authenticates that users session and pulls the image for that interface, so my clients don't know where the server is at. We can do 95% lines and the works by passing different commands.
Ryan