[mrtg] Counts disappear after a few months

[Fred Zwarts (KVI)]

I have a few graphs in which the out channel is much less than the in 
channel.  As an example I take a graph with statistics of our virus scanner 
for the E-mail. The in channel shows the rate of all mail, the in channel 
shows the rate mail with detected malware. The full rate varies between 5 
and 100 mails per hour, but mail with malware is seen only a few times per 
week. (The count is so low, because there is a front-end system that already 
filters out most of the malware.)
So, the daily graph usually shows no malware or one or two spikes for the 
malware.
The weekly graph shows a little bit of "grass" for the malware.
The monthly graph shows some more "grass".
The problem is the yearly graph. It shows some "grass" only for about the 
last 60 days. The grass disappears after about 60 days.
I took a snapshot of the graphs on January 1st. The yearly graph created on 
January 1st shows the "grass" for the months December and November. The 
yearly graph created yesterday, shows the "grass" for the months March, 
February and the last week of January. But the "grass" in December and 
November disappeared, although the scale of the graph did not change. (The 
scale of the graph is determined by the rate of all mail, which had a 
maximum last June.)
Apparently, MRTG looses counts. Is this a known bug? We run MRTG 2.17.2.