Symon is a useful tool that collect data like cpu/bandwidth/whatever usage and display it with nice graphs, like this for example:
First of all, install symon! Since it haven’t configuration options you can use pkg_add -r symon or go to /usr/ports/sysutils/symon and run make install clean.
After a succesful installation, you have to copy your default symon.conf and symux.conf to etc/ directory.
If you have installed symon with the default prefix (/usr/local) you have simply to do
cp /usr/local/share/examples/symon/sym* /usr/local/etc/
Now edit the configuration files, symon.conf and symux.conf.