digraph Perl
{
graph [ rankdir="TB" ]
node [ shape="oval" ]
edge [ color="grey" ]

subgraph "cluster_1"
{
graph [ bgcolor="cornflowerblue" label="Locale" rankdir="TB" ]
node [ shape="oval" ]
edge [ color="grey" ]

"Solar system"
"Earth"
}

subgraph "cluster_2"
{
graph [ bgcolor="lightblue" label="Creatures" rankdir="TB" ]
node [ shape="oval" ]
edge [ color="grey" ]

"Great"
"Small"
}
"Solar system" -> "Earth" [ arrowhead="diamond" label="" ]
"Great" -> "Small" [ arrowhead="odot" label="" ]
}