digraph example {
    rankdir=LR;
    bgcolor="#ffffaa";
    node [fontname=Arial, fontsize=10];
    edge [color=red, style=bold, fontsize=9];

    // Actors
    {
        node [shape=custom, shapefile="Actor.png",
            width=.352, height=0.77, fixedsize=true,
            color="#ffffaa", label="\n\n\n\n\n\n\N"]

        Shark
        Submariner
        Pirate
        Merchant
    }

    // System Use Cases
    subgraph clusterSystem
    {
        label="Sea"; // Name your system here
        fillcolor="#ffffdd"; style=filled; color=black; // The system box
        node [shape=ellipse, style=filled, fillcolor="#ccffcc", color="#558855"]

        Batism
        Inmersion
        "Eat pirates" // You can name it with spaces
        // But is better to use a 'label' property to make easier the references
        Tresour [label="Find a tresour"]
        Attack [label="Attack ships"]
        Escafandrarse [label="Put on the\nescafandre"] // Multiline
        Ron [label="Drink Ron"]
        SellGoods
        Navigate
        GaleonPiloting [label="Navigate in galeon"]
        BuyGoods [label="Buy Goods"]
    }

    // Uses
    Submariner -> Inmersion
    Submariner -> Batism
    Pirate -> Attack
    Pirate -> Ron
    Merchant -> SellGoods
    Shark -> "Eat pirates"

    // Extensions
    {
        edge [label="<<extends>>", dir=back]

        Inmersion -> Tresour
    }

    // Inclusions
    {
        edge [label="<<includes>>"]

        Inmersion -> Escafandrarse
        SellGoods -> Navigate
        SellGoods -> BuyGoods
        Attack -> GaleonPiloting
    }

    // Generalizations
    {
        edge [arrowsize=1.5, arrowtail=empty, dir=back, constraint=false]

        Inmersion -> Batism
        Navigate -> GaleonPiloting
    }
}