This plugin can be download here: errorimage-v0.2-1.tgz
When a Cacti Graph does not exist, this plugin will replace the broken link with a size appropriate Error Image.
Well, the purpose says it all.
Cacti 0.8.7e++ PIA 2.5++ php-gd Module for Resizing Images
Like any other plugin. Install from Plugin Management
There is nothing to learn. It just works.
If you need additional help, please goto http://forums.cacti.net