When creating a new storage volume, the graphs are not created and result in "broken images" until the Nagios service has been restarted manually. It seems as if the oA systemd tries to perform a "reload" action, which does exist on EL7:
The correct fix would be to improve the Nagios systemd service file to support reloads, like it was the case with the SysV init script. This is already tracked as Bug 1084934 on the Red Hat Bugzilla, but still unresolved.
For now, we could resort to restarting Nagios instead of reloading it if we receive a non-zero return value from the reload action.