graph subscribers for ASR9k BNGs(Work in Progress)

I wrote some code to poll the number of active subscribers on a Cisco ASR9k with BNG-function enabled from the CISCO-SUBSCRIBER-SESSION-MIB.
What I have done so far:
added mib in includes/definitions/os.inc.php for cisco devices.
created includes/polling/graphs/cisco-subscriber-sessions-mib.inc.php (http://p.ip.fi/wUMT)
created html/includes/graphs/device/asr9k_bng_sessions.inc.php
added graph in includes/definitions/graphtypes.inc.php

I still need the discovery module to set $graphs_db['asr9k_bng_sessions']) to TRUE somewhere. I try to find how this is done for graphs_db['firewall_sessions_ipv4'] as this is the code I used as template but cant seem to find where graphs_db['firewall_sessions_ipv4'] get set to TRUE in during discovery?
any hints?