One Suggestion for NULL Interface

Hi,
In my Cisco router I using rate-limit command on my subinterfaces, for all my clients group. I using MRTG and graphics from MRTG and NFA are identical with 40-50 Kbps differents. I understand where is the loss this 40-50 Kbits. This is the dropped packets (in NetFlow - destination interface is NULL).
In MRTG I display traffic that clients received, in NFA I display traffic that router received for this clients.
I HAVE ONE SUGGESTION:
Added option in IP Group Management include or exclude NULL interface.
or add on graphics visualization in other colours traffic without NULL interface.

Thanks

Emil