title: 'GridSearchCV' object has no attribute 'tree_' error using graphviz
body: <p>I'm using a gridsearchCV to set parameters for a decision tree regressor as below.</p>
  
  <pre><code>    clf = GridSearchCV(DecisionTreeRegressor(random_state=99),parameters,refit=True,cv=5)  # default is MSE
      clf.fit(x_train, y_train)
  </code></pre>
  
  <p>I then want to pass this output a chart using Graphviz</p>
  
  <pre><code>    export_graphviz(clf, out_file='tree.dot',feature_names=df_temp.columns,leaves_parallel=True)
      subprocess.call(['dot', '-Tpdf', 'tree.dot', '-o' 'tree.pdf'])
  </code></pre>
  
  <p>I'm getting an error message however:
  'GridSearchCV' object has no attribute 'tree_'</p>
  
  <p>I'm really stuck with this and could do with some help please!</p>