pfSense® software has many built-in graphs that monitor different aspects of the system, and they work out-of-the-box with no intervention.
The firewall collects and maintains data about how the system performs, and then stores this data in Round-Robin Database (RRD) files. Graphs created from this data are available under Status > Monitoring.
These graphs measure things such as CPU usage, memory usage, state table usage, throughput (in bytes as well as packets), link quality, traffic shaping queue usage, and more.
The graph on that page can be configured to show items from several categories, and a category and graph may be chosen for both the left axis and right axis for easy comparison.