It's best to use historical monitoring information when all of it is available from the same place and on the same timescale. Most monitoring systems are designed for generic purposes, while allowing application and system developers to integrate their specific checks with the monitoring infrastructure. This is possible through a plugin architecture. Adding new kinds of data inputs to them means installing a plugin. Sometimes, you may need to write or develop this plugin, but writing a plugin for something such as Cacti is easy. You just have to write a script that outputs monitored values in simple text format.
In most common scenarios, the monitoring system is centralized and data is collected directly (and remotely) by the system itself or through some distributed components that are responsible for sending the observed metrics back to the main node.
As far as PostgreSQL is concerned, some useful things to include in graphs are the number...