Network monitoring tool -bandwidth usage

We have a private VPL circuit between 2 sites; both sites are using Fortigate 80C's firewalls for layer 3.  This is a new circuit and for some reason we are having WAY more data than expected crossing the circuit.  I contacted Fortinet to see if they have a bandwidth monitoring tool so we can find out where all of this data is coming from.  I was hoping they had a similar tool to Sonicwalls Viewpoint.  But no such luck; their only solution is the Fortianalyzer device which means I would need to purchase additional hardware.

Any recommendations on an effective software solution that would allow us to monitor bandwidth usage per IP address across the circuit?

Thanks for your help.