title: How to draw parallel edges in Networkx / Graphviz
body: <p>I am trying to add parallel edges between two nodes using NetworkX but it fails with the below error. What am I doing wrong?</p>
  
  <pre><code>import networkx as nx
  import graphviz
  
  g1 = nx.MultiGraph()
  
  node1 = 'a'
  node2 = 'b'
  
  g1.add_edge(node1,node2,key='one')
  g1.add_edge(node1,node2,key='two')
  
  A = nx.to_agraph(g1)
  A.add_subgraph()
  
  A.draw('test2.png', prog='dot')
  </code></pre>
  
  <p>Error:</p>
  
  <pre><code>Traceback (most recent call last):
    File "test2.py", line 12, in &lt;module&gt;
      A = nx.to_agraph(g1)
    File "C:\python27\lib\site-packages\networkx-1.11rc1-py2.7.egg\networkx\drawing\nx_agraph.py", line 152, in to_agraph
      A.add_edge(u,v,key=str(key),**str_edgedata)
    File "C:\python27\lib\site-packages\pygraphviz\agraph.py", line 481, in add_edge
      eh = gv.agedge(self.handle, uh, vh, key, _Action.find)
  KeyError: 'agedge: no key'
  </code></pre>