title: Graphviz subgraph doesn't get visualized
body: <p>I'm trying to create a graph with two subgraphs in dot. The code is as follows:</p>
  
  <pre><code>digraph G {
          subgraph step1 {
                  style=filled;
                  node [label="Compiler"] step1_Compiler;
                  node [label="Maschine"] step1_Maschine;
                  color=lightgrey;
          }
  
          subgraph step2 {
                  style=filled;
                  color=lightgrey;
                  node [label="Interpretierer"] step2_Interpretierer;
                  node [label="Maschine"] step2_Maschine;
                  label="Virtuelle Maschine";
          }
  
          "Programm (Java)" -&gt; step1_Compiler;
          step1_Compiler -&gt; step1_Maschine;
          step1_Maschine -&gt; "Bytecode";
          "Bytecode" -&gt; step2_Interpretierer;
          step2_Interpretierer -&gt; step2_Maschine;
          step2_Maschine -&gt; "Ergebnis";
  }
  </code></pre>
  
  <p>The result I am getting looks like the following:</p>
  
  <p><img src="https://i.stack.imgur.com/2xbz0.png" alt="Result of above code"></p>
  
  <p>I expected to see a box around both subgraphs. What am I missing here?</p>