digraph {
    graph [
        fontname="Helvetica-Oblique"
        rankdir="TB"
    ]

    node [
        shape="circle";
    ]
    {rank="same"; 0; 3}

    0 [label="b"]
    1 [label="a"]
    2 [label="d"]

    0 -> 1
    1 -> 6

    3 [label="d"]
    4 [label="a"]
    5 [label="d"]

    0 -> 3 [style="dotted"]
    3 -> 4
    4 -> 5

    6 [label="b"]
    7 [label="y"]

    {rank="same"; 2; 6}
    6 -> 2 [style="dotted"]
    6 -> 7

    8 [label="n"]
    9 [label="c"]
    10 [label="e"]

    {rank="same"; 5; 8}
    5 -> 8 [style="dotted"]
    8 -> 9
    9 -> 10

    11 [label="o"]
    12 [label="x"]

    {rank="same"; 1; 11}
    1 -> 11 [style="dotted"]
    11 -> 12

    13 [label="n"]
    14 [label="k"]

    {rank="same"; 6; 13}
    2 -> 13 [style="dotted"]
    13 -> 14;
}