Why is my graph/rrd zeroed, when snmpwalk shows data?

I have a graph, monitoring a enviromental sensor box. For a few weeks
my graphs have been blank, and I see only 0 in the rrd. But when I use
snmpwalk to check, I get real numbers.

Anyone have some suggestions for how to debug something like that?
Where is the data going? Where am I getting these zeroes?

-andreas

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