Flat graphs when using long time periods

We're using cacti 0.8.8a with rrdtool 1.3.8
We've been using cacti for about 7 years.

In a few graphs, when I select a longer period of time,
like a year, there is a flat line.  Most graphs look
reasonable if I extend the date span to one year.

If I reduce the date preset to something like 2 months,
I get the bumps and valleys I expect to see.

The setting on Associated RAA's is Daily 5 min avg.

The graphs where this is happening use custom snmp
queries via extend.

What is this symptomatic of?

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