digraph {
    rankdir=TB
    node [shape="rectangle"];

    /* top two layers form an undirected graphical model */
    subgraph cluster_top {
        color=white
        rankdir=BT
        h3 [label="Hidden layer 3"]
        h2 [label="Hidden layer 2"]
        h3 -> h2
        h2 -> h3
    }

    h1 [label="Hidden layer 1"]
    v  [label="Visible layer (observed)" style=filled]
    h2 -> h1
    h1 -> v
}