A few things:
1) This is on the Dashboard page? On the Services or any of the Host pages, do you see the graphs?
2) Those graphs are generated based on the metrics data collected in Ganglia. So it’s not related to Nagios.
3) What is the time difference between the hosts in your cluster? All running ntp so time is in-sync? How about difference between your local machine (where you are accessing via browser) and the Ambari server host?