After successfully using Nagiosgraph since a couple of days now, I experienced a weird behavior in the statistic graphs. Suddenly some gaps (or empty spaces, missing data) showed up, the graphic line didn't continue:

I marked the gaps with the red oval. Take a look at it and you notice the missing continous blue line. Ugly, isn't it?

Why did this happen? Actually, it's my own fault. I scheduled some Nagios checks to be done only once an hour, some every 15mins, some every minute, and so on (different service objects). Now Nagiosgraph has a so-called Heartbeat which is by default set to 600 (seconds). So if the rrd file isn't updated within the 600s (= 10min) the time-span without data would be declared as UNKNOWN and this results in the empty data graphic/gap.

To change this the heartbeat you need to open your nagiosgraph.conf and change the heartbeat variable to be bigger than your longest check period. And one more note: You need to delete your already created rrd file afterwards; the new settings (the new heartbeat value) is only applied on new generated rrd files.