Cacti is a GUI wrapper for the RRDTool program, and is used to create graphs of network and other system activity.
Monitoring PostgreSQL Databases
Currently there is an early Host Template for Cacti available from their forums. It's also possible to use the check_postgres monitoring tool with cacti; see Cacti + PostgreSQL Graphing for notes on doing that.
Supporting PostgreSQL data storage
Cacti is written in PHP and supports MySQL only for storing the RRDTool data. There is a generic bug report named Support Multiple Databases for Cacti Engine. There seems to be support from the users as well as the developers to having Postgres support: