title: Visualization of a decision tree inside a pipeline
body: <p>I would like to visualize my decision tree with <code>export_graphviz</code>, however I keep on getting the following error:</p>
  
  <pre class="lang-py prettyprint-override"><code>
    File "C:\Users\User\AppData\Local\Continuum\anaconda3\envs\data_science\lib\site-packages\sklearn\utils\validation.py", line 951, in check_is_fitted
      raise NotFittedError(msg % {'name': type(estimator).__name__})
  
  NotFittedError: This Pipeline instance is not fitted yet. Call 'fit' with appropriate arguments before using this method.
  </code></pre>
  
  <p>I am pretty sure my Pipeline is fitted because I call predict in my code which works just fine. Here is the code in question:</p>
  
  <pre class="lang-py prettyprint-override"><code>from sklearn.tree import DecisionTreeRegressor
  import pandas as pd
  import numpy as np
  from sklearn.pipeline import Pipeline
  from sklearn.model_selection import train_test_split
  from sklearn.tree import export_graphviz
  
  #Parameters for model building an reproducibility
  state = 13
  
  data_age.dropna(inplace=True)
  X_age = data_age.iloc[:,0:77]
  y_age =  data_age.iloc[:,77]
  
  X = X_age
  y = y_age
  
  #split between testing and training set
  X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.2, random_state= state)
  
  # Pipeline with the regressor
  regressors = [DecisionTreeRegressor(random_state = state)]
  for reg in regressors:
      steps=[('regressor', reg)]
      pipeline = Pipeline(steps) #seed that controls the random grid search
  
  #Train the model
  
  pipeline.set_params(regressor__max_depth = 5, regressor__min_samples_split =5, regressor__min_samples_leaf = 5).fit(X_train, y_train)
  pred = pipeline.predict(X_test)
  pipeline.score(X_test, y_test)
  
  export_graphviz(pipeline, out_file='tree.dot')
  </code></pre>
  
  <p>I know I don't really need the Pipeline here but I would still like to understand what is the problem for future reference and be able to plot a decision tree, whithin a pipeline which has been fitted.</p>