You said "graphical" in the subject, so
Zabbix (client) is free.
Zabbix (server) is free.
snmp is free.
http://collectl.sourceforge.net/ is free.
but this has no graphical front-end, so that would be up to you.
Also, use of "host" and "target" is ambiguous in a client-server model.
Either way, zabbix can do this with it's agent
and snmp collection.
Custom metrics would have to be scripted.
Other solutions may be out there.
Wait for additional suggestions.
Have a good day.