title: How do I add edges between two subgraphs in pydot?
body: <p>Does anyone know how to add an edge between two subgraphs (clusters) in pydot?</p>
  
  <pre><code>callgraph = pydot.Dot(graph_type='digraph',fontname="Verdana")
  cluster_foo=pydot.Cluster('foo',label='foo')
  cluster_foo.add_node(pydot.Node('foo_method_1',label='method_1'))
  callgraph.add_subgraph(cluster_foo)
  
  cluster_bar=pydot.Cluster('bar',label='Component1')
  cluster_bar.add_node(pydot.Node('bar_method_a'))
  callgraph.add_subgraph(cluster_bar)
  </code></pre>
  
  <p>I tried:</p>
  
  <pre><code>callgraph.add_edge(pydot.Edge("foo","bar"))
  </code></pre>
  
  <p>but doesn't work. It just creates two more nodes labeled "foo" and "bar" in the initial graph and puts an edge between them!</p>
  
  <p>Can anyone help, please?</p>