Grafana fetches data from AMS collector which runs n default port 6188. So can you please check if From the Grafana Host ambari Metrics Collector Host&port is reachable or not?
Example: From Grafana Host run the following, It will help in isolating two things
1. Grafana Host is able to resolve the hostname "compute-1-6.local" properly. (checking the /etc/hosts entry will help)
2. There is no Firewall issue which is restricting access on port 6188
telnet compute-1-6.local 6188
And from Ambari Metrics Collector Host check if it has the correct hostname and port is opened?
# echo "On AMS host"
netstat -tnlpa | grep 6188