SubGraph

This allows you to define a graph within the graph. This will group those objects together in some engines.

 graph {
    subgraph -Attributes @{label='DMZ'} {
        node web1,web2,reports
        edge report -To web1,web2
    }        
    edge loadBalancer -To web1,web2
    edge user -To loadBalancer,report
    edge web1,web2,report -To database
}

Source