Define "generic" graphs not attached to hosts. Graph items from different hosts (currently, when defining a graph with items from different hosts, the graph is saved to on the host of the first item in the graph), attach/save graph on different host that any of the hosts of the items graphed (i.e. define a virtual host, not monitored then define graphs attached to this virtual host that draw items from other hosts).
Another way to do this is to link items: define virtual host, not monitored, define items attached to this host that point to items from different hosts, the graph the virtual host items.
Also, in graph definition (or item definition) function to sum/average/min/max from all similar items in the host group or application without manually specify each one.
Use case: Application providing a service, deployed distributed on several hosts. All instances of the application have same statistics but is very time consuming and difficult to graph all instances in the same graph, overlaid (to spot differences in service request distribution over all application instances) or summed automatically (to have an overview of the total service requests handled together by all application instances).