Cacti users sometimes complain about NaN's in their graphs. Unfortunately, there are several reasons for this result. The following is a step-by-step procedure recommended for debugging.
Please have a look at your cacti log file. Usually, you'll find it at <path_cacti>/log/cacti.log. Else see Settings, Paths. Check for this kind of error:
CACTID: Host[...] DS[....] WARNING: SNMP timeout detected [500 ms], ignoring host '........'
For "reasonable" timeouts, this may be related to a snmpbulkwalk issue. To change this, see Settings, Poller and lower the value for The Maximum SNMP OID's Per SNMP Get Request. Start at a value of 2 and increase it again, if the poller starts working. (1 or less disables snmpbulkwalk) Some agent's don't have the horsepower to deliver that many OID's at a time. Therefore, we can reduce the number for those older/underpowered devices.