title: parse multiple digraph in one dot file
body: <p>I'm trying to process and render some graph in DOT format.
  The dot file I have is large (~300MB), and it contains multiple digraphs  </p>
  
  <pre><code>digraph 1 {...}
  digraph 2 {...}
  digraph 3 {...}
  </code></pre>
  
  <p>I have got 2 questions:<br>
  1. Is it possible to use render only 1 digraph instead of the whole graph?<br>
  Something like <code>dot -3 -Tps mygraph.dot -o out.ps</code> to render digraph 3 only?</p>
  
  <p>2.What's the best Python library to process dot format?(other languages are also acceptable)<br>
  Here are two I tried, but not good enough<br>
  <strong>pydot</strong> It gives me a digraph list after importing, which is good, but it doesn't handle "." in the node name. For example <code>nd.nd [label="nd_node"]</code> will fail</p>
  
  <p><strong>pygraphviz</strong> It does handle ".", but only imports digraph 1 when given multiple graph definition in a file :(</p>