使聚类graphviz图看起来更像树

Question

我有以下Graphviz：

graph {
    node[width = 0.6, height = 0.6, fixedsize=true, shape=circle];
    nodesep = 0.5;
    a[label="22"];
    b[label="22"];
    c[label="34"];
    d[label="22"];
    e[label="99"];
    f[label="34"];
    g[label="40"];
    h[label="37"];
    i[label="22"];
    j[label="99"];
    k[label="135"];
    l[label="129"];
    m[label="40"];
    edge[penwidth=1.0]; //principal path edges
    a -- b;
    b -- d;
    c -- f;
    d -- i;
    e -- j;
    g -- m;
    edge[penwidth=1.0]; //other edges
    a -- c;
    b -- e;
    c -- g;
    d -- h;
    e -- k;
    g -- l;
}

这给我画了一个漂亮的树形图。然后，我想更改一些内容（特别是＆＃39;绘制具有匹配值的路径），所以我这样做了，following this example：

graph {
    node[width = 0.6, height = 0.6, fixedsize=true, shape=circle];
    nodesep = 0.5;
    subgraph cluster_0 {
             style = rounded;
             a[label="22"];
             b[label="22"];
             d[label="22"];
             i[label="22"];
    }
    subgraph cluster_1 {
             style = rounded;            
             c[label="34"];
             f[shape=diamond, label="34"];
    }
    subgraph cluster_2 {
             style = rounded;
             e[label="99"];
             j[shape=diamond,label="99"]; 
    }
    subgraph cluster_3 {
             style = rounded;
             g[label="40"];
             m[shape=diamond, label="40"];
    }
    node[shape = diamond];
    h[shape=diamond, label="37"];
    k[label="135"];
    l[label="129"];
    edge[penwidth=1.0]; //principal path edges
    a -- b;
    b -- d;
    c -- f;
    d -- i;
    e -- j;
    g -- m;
    edge[penwidth=3.0]; //other edges
    a -- c;
    b -- e;
    c -- g;
    d -- h;
    e -- k;
    g -- l;
}

但是，最终结果是每个群集都是垂直的，看起来并不像树一样。有没有办法让第二张图看起来更像第一张？