title: top-down subgraphs, left-right inside subgraphs
body: <p>I'd like to have my graph looks like this:</p>
  
  <p><img src="https://i.imgur.com/W4aZG.jpg" height="500" /></p>
  
  <p>But I can only get this:</p>
  
  <p><img src="https://i.imgur.com/p2ap0.png" height="500"/></p>
  
  <p>The problem is, <code>rankdir</code> <a href="http://www.graphviz.org/bugs/b1279.html" rel="noreferrer">does not work</a> in <code>subgraph</code>. 
  So, how to emulate it?</p>
  
  <p>The code:</p>
  
  <pre><code>digraph G {
      node [shape = circle]
      0 [style = invis]
  
      0 -&gt; "0A"
  
      subgraph clusterA {
          label=A
          "0A"
          "1A"
          "2A" -&gt; "0A" [label=•]
      }
  
      subgraph clusterB {
          label=B
          "0B"
          "1B"
          "2B" -&gt; "0B" [label=•]
      }
  
          subgraph clusterC {
          label=C
          "0C"
          "1C"
          "2C" -&gt; "0C" [label=•]
      }
  
      subgraph clusterD {
          label=D
          "0D"
          "1D"
          "2D" -&gt; "0D" [label=•]
      }
  
      subgraph clusterE {
          label=E
          "0E"
          "1E"
          "2E" -&gt; "0E" [label=•]
      }
  
      subgraph clusterF {
          label=F
              {node [shape = doublecircle] "0F" "1F"}
          "2F" -&gt; "0F" [label=•]
      }
  
      "0A" -&gt; "1B" [label=a]
      "1A" -&gt; "2B" [label=a]
      "0B" -&gt; "1C" [label=b]
      "1B" -&gt; "2C" [label=b]
      "0C" -&gt; "1D" [label=c]
      "1C" -&gt; "2D" [label=c]
      "0D" -&gt; "1E" [label=d]
      "1D" -&gt; "2E" [label=d]
      "0E" -&gt; "1F" [label=e]
      "1E" -&gt; "2F" [label=e]
  }
  </code></pre>