MRTG bandwidth not accurate

MRTG is great for pulling counters from network interfaces (and temperatures, and CPU usage, and other things, too). If a device doesn't provide an SNMP-accessible counter for what you want to measure, though, MRTG won't be a lot of help.

In the case of a Cisco PIX, no SNMP counters are provided in its MIB for "sessions". The PIX is capable of reporting the setup and take-down of NAT table entries (including the number of bytes moved in the "conversation") via SYSLOG.

A tool that I have no personal experience with, but that looks interesting, is FirePlotter.

I do have some experience with the PIX Logging Architecture open source project, which gets its data from SYSLOG output from PIX / ASA firewalls. Its database schema doesn't capture the bytes transferred from the log entries, though, so you can't do any reporting on bandwidth usage. (It would probably be fairly easy to add, but their database choice, MySQL, turned me off to the idea of helping out with their project...)

This is absolutely possible. The text on and around the graphs is fully themeable. The documentation shows this pretty well. For example, to change the "Traffic Analysis for xyz" text use the key:

Title[GigE2/1]: Description of switch port GigE2/1

If you're generating your MRTG graphs by way of cfgmaker

cfgmaker --ifdesc=descr $OtherOptions

Should render you the same thing automatically. Your titles won't be Traffic analysis for 2/1 they will be Traffic analysis for Payments Server, which sounds like what you're looking for. If you need something more then that, then you need to start looking into the "--if-template" function of cfgmaker. It allows a much more detailed customization of the text fields, but does require some scripting.