title: group nodes with subgraphs
body: <p>I'd like to group some nodes with the following code</p>
  
  <pre><code>digraph dataflow {
      subgraph pipeline {
          relations;
          synonyms;
          articles;
      }
      subgraph lucene {
          index;
          search;
      }
      training_data - &gt; index;
      relations - &gt; search;
      synonyms - &gt; index;
      articles - &gt; index;
      training_data - &gt; evaluation;
  }
  </code></pre>
  
  <p>But dot doesn't care about the subgraphs:</p>
  
  <p><img src="https://i.stack.imgur.com/JFitD.png" alt="example dot graph"></p>