Mukundh,
You can use PDM which is the gui interface, and under monitoring you can pool some interface graphs from there , this is if your pix is still running 6.x or prior codes and PDM , see graphs bellow
http://www.cisco.com/en/US/docs/security/pix/pix63/pdm30/olh/PDF_Version_-_PDM_OLH/pdm30olh.pdf
If under version code 8x use ASDM gui interface graphs section
http://www.cisco.com/en/US/partner/docs/security/asa/asa83/asdm63/configuration_guide/intrface.html#wp1345760
You can also use 3rd party such as prtg and monitor eirther the interfaces the firewalls are connected to or firewalls interfaces via snmp , there is a prtg freeware out there not sure if still freeware , you can google it.
http://www.paessler.com/prtg
Regards
Jorge Rodriguez
