When viewing a graph for a remotely monitored device, there are no datapoints seen and the hostname under associated device is duplicated:
 

On the agent doing the remote monitoring, execute following command:

nslookup <remote device hostname>

If 'Name:' is duplicated in the output due to IPv4 and IPv6 both being enabled then this is the cause of the problem.

The workaround is to modify the remote monitoring policy so that the remote server name is in format:

<hostname>;<publish_hostname>

e.g.

raspberrypi;raspberrypi.home