If you are only interested in overall per-interface usage, there are built-in RRD graphs on pfSense, which can be found underStatus > RRD Graphs.
If you need a little more detail, such as by client IP on your LAN interface, there is a package for bandwidthd that you can install under System > Packages. Once installed, it appears under Services > BandwidthD.
Darkstat is also available in System > Packages. Once installed, it appears under Diagnostics > darkstat. It also offers bandwidth graphs for an interface, as well as traffic to/from specific IP addresses.