title: MGLEARN Plot KNN classification doesn't show the plot
body: <p>I am moving the first ML steps reading the book <a href="https://books.google.it/books?id=1-4lDQAAQBAJ&amp;printsec=frontcover&amp;dq=Introduction%20to%20Machine%20Learning&amp;hl=it&amp;sa=X&amp;ved=0ahUKEwjv8YqiodjXAhXkKsAKHQ_PAPAQ6wEITzAF#v=onepage&amp;q=Introduction%20to%20Machine%20Learning&amp;f=false" rel="nofollow noreferrer">Introduction to Machine Learning</a>.
  I am trying to generate the picture of the snippet "<strong>In [10]</strong>" that you can find on <strong><a href="https://github.com/amueller/introduction_to_ml_with_python/blob/master/02-supervised-learning.ipynb" rel="nofollow noreferrer">this</a></strong> page, but it down't work. When I say that it doesn't work I mean that nothing it's been shown when I hit "run" (neither an error message). </p>
  
  <p>What's wrong with the following code?</p>
  
  <p>I think that I am missing something like plt.show() code but looking on Google it seems that mglearn doen't need/have this construction.</p>
  
  <pre><code>from sklearn.neighbors import KNeighborsClassifier
  import matplotlib.pyplot as plt
  import mglearn.plots
  
  X, y = mglearn.datasets.make_forge()
  mglearn.discrete_scatter(X[:, 0], X[:, 1], y)
  plt.legend(["Class 0", "Class 1"], loc=4)
  plt.xlabel("First feature")
  plt.ylabel("Second feature")
  print("X.shape: {}".format(X.shape))
  #plt.show()
  
  
  mglearn.plots.plot_knn_classification(n_neighbors=1)
  </code></pre>
  
  <p>I am using Python 3.6.3.</p>