title: Sckit learn with GraphViz exports empty outputs
body: <p>I would like to export decision tree using sklearn.</p>
  
  <p>First I trained a decision tree classifier:</p>
  
  <pre><code>self._selected_classifier = tree.DecisionTreeClassifier()
  self._selected_classifier.fit(train_dataframe, train_class)
  
  self._column_names = list(train_dataframe.columns.values)
  </code></pre>
  
  <p>After that I used the following method in order to export the decision tree:</p>
  
  <pre><code>def _create_graph_visualization(self):
      decision_tree_classifier = self._selected_classifier 
  
      from sklearn.externals.six import StringIO
      dot_data = StringIO()
      tree.export_graphviz(decision_tree_classifier,
                           out_file=dot_data,
                           feature_names=self._column_names)
      import pydotplus
      graph = pydotplus.graph_from_dot_data(dot_data.getvalue())
      graph.write_pdf("decision_tree_output.pdf")
  </code></pre>
  
  <p>After many errors regarding missing executables now the program is finished successfully. 
  The file is created, but it is empty. 
  What am I doing wrong? </p>