digraph abc {
    node [shape=circle, size=0.2];
    rankdir=LR;

    x₁ -> a₁[label = w¹];
    x₁ -> {a₂ a₃};
    x₂ -> {a₁ a₂ a₃};

    a₁ -> d₁[label = w²];
    a₁ -> d₂;
    a₂ -> {d₁ d₂};
    a₃ -> {d₁ d₂};

    subgraph {rank=source; x₁; x₂};
    subgraph {rank=same; a₁; a₂; a₃};
    subgraph {rank=sink; d₁; d₂};
}