The plugin must return performance data for it to be graphed. Check that
the performance data appears in the Nagios GUI
(
http://diskstation:8888/nagios3/). Click on the service and make sure
there is some data in the Performance Data field. Mine is like this for CPU:
Performance Data: 'user'=2.40 'nice'=0.55 'sys'=3.15 'softirq'=1.07
'iowait'=24.30 'irq'=0.50 'idle'=71.49
Also check the Synagios log in the DSM package manager. It should show
all the processes that are running, and one of them should be npcd,
which is responsible for processing the performance data. Npcd should be
listed in the log as '/usr/sbin/npcd -d -f /etc/pnp4nagios/npcd.cfg'.
Hope this helps!