[mrtg] strange degradation problem

[Peter]

Hi!

Check out http://www.fiber.nu/mrtg/index.html
i have put up 2 images from mrtg, one taken from Saturday, and one from 
Sunday, the strange thing is that the graph on Friday forexample is much 
larger in the first image than the second, and it's the same thing in 
the yearly graphs, stuff older than 2month is just zero. what could 
possible cause the values to diminish like this?

(Debian/mrtg 2.14.7-2)