The MRTG addon is showing how many viruses that has ben caught by Clam not a how many viruses you have in the "quarantine".
As you can read in the statistic collection script /etc/mrtg/virus-stats, mrtg get's the data from the clam log file /var/log/amavis-ng/amavis-ng.log
I guess that you can always delete the log file /var/log/amavis-ng/amavis-ng.log to reset the mrtg data byt then you will also lose the history.
But maybee you can file this as an request to Jesper to do this as an addition to his package ?
/Mats