Easiest way is to dump call detail records to either a AAA server or a syslog server, parse the logs and use another application to produce the graphs.
You can query the inbuilt SNMP MIBS as well, but would need some kind of network management application.
Here is a note on getting Call Detail Records to a syslog server -
http://www.cisco.com/warp/customer/788/voip/cdr_logging.html