title: Error "GraphViz's executables not found" when calling GraphViz layout from NetworkX in iPython notebook
body: <p>I received the error "InvocationException: GraphViz's executables not found, error GraphViz's executables not found" when trying to use the method graphviz_layout in my code.  The code works fine when graphviz_layout is replaced by spring_layout.  From the throwback exception information, it looks like it calls pydot_layout.  I have the pydot module installed through Canopy's package manager, but importing it does not help.  I have also pip installed graphviz and imported it with no luck.</p>
  
  <p>Here is my code:</p>
  
  <pre><code>import networkx as nx
  import matplotlib.pyplot as plt
  
  keywordTreeFile = open('decode_wordnet/keywordTreeFile.TXT','r') #keyword generation file
  keywordTreeFileLineData = keywordTreeFile.readlines()
  
  G = nx.Graph()
  pairData = []
  
  for i in range(0,len(keywordTreeFileLineData)):
      pairData = pairData + [keywordTreeFileLineData[i].split('\t')]
      pairData[i][1] = pairData[i][1].rstrip('\n')
      G.add_edge(pairData[i][0],pairData[i][1])
  
  pos = nx.graphviz_layout(G)
  
  nx.draw(G,
          pos=pos,
          width = 1.0,
          with_labels = True,
          font_size = 3,
          linewidths=.1
          )
  plt.savefig("graph.pdf")
  </code></pre>
  
  <p>Here is the throwback exception information:</p>
  
  <pre><code>---------------------------------------------------------------------------
  InvocationException                       Traceback (most recent call last)
  &lt;ipython-input-1-e85a11cf6191&gt; in &lt;module&gt;()
       15 
       16 #pos = nx.spring_layout(G,k=.15,iterations=50,scale=100)
  ---&gt; 17 pos = nx.graphviz_layout(G)
       18 #labels = nx.draw_networkx_labels(G,pos)
       19 nx.draw(G,
  
  /Users/scott/Library/Enthought/Canopy_64bit/User/lib/python2.7/site-packages/networkx/drawing/nx_pydot.pyc in graphviz_layout(G, prog, root, **kwds)
      245     This is a wrapper for pydot_layout.
      246     """
  --&gt; 247     return pydot_layout(G=G,prog=prog,root=root,**kwds)
      248 
      249 
  
  /Users/scott/Library/Enthought/Canopy_64bit/User/lib/python2.7/site-packages/networkx/drawing/nx_pydot.pyc in pydot_layout(G, prog, root, **kwds)
      269         P.set("root",make_str(root))
      270 
  --&gt; 271     D=P.create_dot(prog=prog)
      272 
      273     if D=="":  # no data returned
  
  /Users/scott/Library/Enthought/Canopy_64bit/User/lib/python2.7/site-packages/pydot.pyc in &lt;lambda&gt;(f, prog)
     1800             self.__setattr__(
     1801                 'create_'+frmt,
  -&gt; 1802                 lambda f=frmt, prog=self.prog : self.create(format=f, prog=prog))
     1803             f = self.__dict__['create_'+frmt]
     1804             f.__doc__ = '''Refer to the docstring accompanying the 'create' method for more information.'''
  
  /Users/scott/Library/Enthought/Canopy_64bit/User/lib/python2.7/site-packages/pydot.pyc in create(self, prog, format)
     1951             if self.progs is None:
     1952                 raise InvocationException(
  -&gt; 1953                     'GraphViz\'s executables not found' )
     1954 
     1955         if not self.progs.has_key(prog):
  
  InvocationException: GraphViz's executables not found
  </code></pre>
  
  <p>System information:  I am using the lastest version and modules of Enthought Canopy on a 64-bit Mac with OS X 10.9.5.  Also, I am using an iPython notebook.</p>