Zabbix graph of item from different hosts

You can create a custom graph that pulls items from multiple hosts. The graph however will be associated with a host, in which case you could create a phantom host which you can associate the graph with. Configuration -> Hosts -> Graphs (in the host you want the graph to be associated with). When you add your Graph items you can selected the host from which each item comes from.

Alternatively there are aggregate hosts available as well. These allow for the accumulation of items across all hosts in a host group.

Since Zabbix 2.4 you may also simply mark such items from the latest data page and display an ad-hoc graph. There is no built-in way to save such graph - but if you would like to revisit it later, you can copy the URL to it. See Zabbix manual for more information : https://www.zabbix.com/documentation/2.4/manual/config/visualisation/graphs/adhoc