figvirtnonvirt.dot

digraph virtnonvirt {
    graph [nodesep=1.5, rankdir=BT];

    B1 [label = "B", shape=plaintext, height=.1, fontsize=11, fontname="Courier New"];
    B2 [label = "B", shape=plaintext, height=.1, fontsize=11, fontname="Courier New"];
    AA [shape=plaintext, height=.1, fontsize=11, fontname="Courier New"];
    X [shape=plaintext, height=.1, fontsize=11, fontname="Courier New"];
    Y [shape=plaintext, height=.1, fontsize=11, fontname="Courier New"];
    Z [shape=plaintext, height=.1, fontsize=11, fontname="Courier New"];

    subgraph {
    AA -> X;
    AA -> Y;
    X  -> B1;
    Y  -> B1;
    }

    AA -> Z [weight=0];
    Z  -> B2;
}