title: fail to visualize a network in python - issue with pygraphviz?
body: <p>As I use some code to visualize a network, I encountered an error</p>
  
  <pre><code>Traceback (most recent call last):
  File "networkplot.py", line 21, in &lt;module&gt;
  pos = graphviz_layout(G, prog='neato')
  File "/opt/conda/lib/python2.7/site-packages/networkx/drawing/nx_agraph.py", line 228, in graphviz_layout
  return pygraphviz_layout(G,prog=prog,root=root,args=args)
  File "/opt/conda/lib/python2.7/site-packages/networkx/drawing/nx_agraph.py", line 258, in pygraphviz_layout
  'http://pygraphviz.github.io/')
  ImportError: ('requires pygraphviz ', 'http://pygraphviz.github.io/')
  </code></pre>
  
  <p>I therefore download <code>pygraphiviz-1.3.1.tar.gz</code> and <code>pip install pygraphviz</code>. It showed </p>
  
  <pre><code>Failed building wheel for pygraphviz
  Running setup.py clean for pygraphviz
  Failed to build pygraphviz
  Installing collected packages: pygraphviz
  Running setup.py install for pygraphviz ... error
  Complete output from command /opt/conda/bin/python -u -c "import setuptools, tokenize;__file__='/tmp/pip-build-0RJB9q/pygraphviz/setup.py';exec(compile(getattr(tokenize, 'open', open)(__file__).read().replace('\r\n', '\n'), __file__, 'exec'))" install --record /tmp/pip-vo59d4-record/install-record.txt --single-version-externally-managed --compile:
  running install
  Trying pkg-config
  Package libcgraph was not found in the pkg-config search path.
  Perhaps you should add the directory containing `libcgraph.pc'
  to the PKG_CONFIG_PATH environment variable
  No package 'libcgraph' found
  Traceback (most recent call last):
    File "&lt;string&gt;", line 1, in &lt;module&gt;
    File "/tmp/pip-build-0RJB9q/pygraphviz/setup.py", line 87, in &lt;module&gt;
      tests_require=['nose&gt;=0.10.1', 'doctest-ignore-unicode&gt;=0.1.0',],
    File "/opt/conda/lib/python2.7/distutils/core.py", line 151, in setup
      dist.run_commands()
    File "/opt/conda/lib/python2.7/distutils/dist.py", line 953, in run_commands
      self.run_command(cmd)
    File "/opt/conda/lib/python2.7/distutils/dist.py", line 972, in run_command
      cmd_obj.run()
    File "setup_commands.py", line 44, in modified_run
      self.include_path, self.library_path = get_graphviz_dirs()
    File "setup_extra.py", line 121, in get_graphviz_dirs
      include_dirs, library_dirs = _pkg_config()
    File "setup_extra.py", line 44, in _pkg_config
      output = S.check_output(['pkg-config', '--libs-only-L', 'libcgraph'])
    File "/opt/conda/lib/python2.7/subprocess.py", line 573, in check_output
      raise CalledProcessError(retcode, cmd, output=output)
  subprocess.CalledProcessError: Command '['pkg-config', '--libs-only-L', 'libcgraph']' returned non-zero exit status 1
  
  ----------------------------------------
  Command "/opt/conda/bin/python -u -c "import setuptools, tokenize;__file__='/tmp/pip-build-0RJB9q/pygraphviz/setup.py';exec(compile(getattr(tokenize, 'open', open)(__file__).read().replace('\r\n', '\n'), __file__, 'exec'))" install --record /tmp/pip-vo59d4-record/install-record.txt --single-version-externally-managed --compile" failed with error code 1 in /tmp/pip-build-0RJB9q/pygraphviz/
  </code></pre>
  
  <p>The code that I used:</p>
  
  <pre><code>import networkx as nx
  import matplotlib.pyplot as plt
  plt.switch_backend('agg')
  from networkx.drawing.nx_agraph import graphviz_layout
  import graphs 
  
  A = graphs.create_graph()
  graph = A.graph
  G, labels = A.networkList()
  
  fig = plt.figure()
  pos = graphviz_layout(G, prog='neato')
  </code></pre>
  
  <p>Could anyone let me know how to solve this? I highly appreciate your assistance. Thank you very much!</p>