I monitor my instances of untangle, as well as the network switches via OpenNMS, and see this kind of graphs, as well as alerts if anything spikes above normal amounts. This is really something better done outside the UTM, since it can go down. This separate box can tell me of an outage, and show near live data (10 seconds old for my network).
Give the attached PDF a look for one of my test untangle boxes.
Generally from Untangle you get network segment usage, then drill down to individual switches downstream to track usage by vlan/port.