From Observium

Make sure the hostname Observium uses matches the hostname the Collectd instances on the servers you want to graph match :

In collectd.conf :

Hostname "alpha.memetic.org"

At the collecting server side (your observium server) add:

LoadPlugin network
<Plugin network>
Listen "0.0.0.0" "25826"
</Plugin>

At all clients add (replace 1.2.3.4 with IP address of your observium server):

LoadPlugin network
<Plugin network>
Server "1.2.3.4" "25826"
</Plugin>

Tell Observium where Collectd is keeping its RRD files in config.php :

$config['collectd_dir']         = "/mnt/rrdcached/db/collectd/";

A 'collectd' tab should automatically appear for any servers that collectd is receiving graphs for (and where the hostnames match)