title: change text in boxes of confusion matrix plot
body: <p>I am trying to use the <a href="https://scikit-learn.org/stable/modules/generated/sklearn.metrics.plot_confusion_matrix.html#sklearn.metrics.plot_confusion_matrix" rel="nofollow noreferrer"><code>plot_confusion_matrix</code></a> function that is available in scikitlearn 0.22. However, I am having an issue where the text values in the box cannot be seen for one of my boxes because all the text colors seem to be set at the same value as that box. This is the case no matter which <code>cmap</code> I select. Incidentally that value is also the lowest one. This does not happen in the <a href="https://scikit-learn.org/stable/auto_examples/model_selection/plot_confusion_matrix.html#sphx-glr-auto-examples-model-selection-plot-confusion-matrix-py" rel="nofollow noreferrer">example they provide</a>. How can I change the box text color so that all the values can be seen clearly? I do not want to use <code>seaborn</code> like has been suggested in many of the other solutions unless I have to.</p>
  
  <p>A reproducible example is below.</p>
  
  <pre><code>import numpy as np
  import matplotlib.pyplot as plt
  
  from sklearn import datasets
  from sklearn.model_selection import train_test_split
  from sklearn.metrics import plot_confusion_matrix
  
  np.random.seed(3851)
  
  # import some data to play with
  bc = datasets.load_breast_cancer()
  X = bc.data
  y = bc.target
  class_names = bc.target_names
  
  # Split the data into a training set and a test set
  X_train, X_test, y_train, y_test = train_test_split(X, y, random_state=0)
  np.random.shuffle(y_test)
  
  # Run classifier, using a model that is too regularized (C too low) to see
  # the impact on the results
  classifier = svm.SVC(kernel='linear', C=0.0001).fit(X_train, y_train)
  
  np.set_printoptions(precision=2)
  
  # Plot non-normalized confusion matrix
  titles_options = [("Confusion matrix, without normalization", None),
                    ("Normalized confusion matrix", 'true')]
  for title, normalize in titles_options:
      disp = plot_confusion_matrix(classifier, X_test, y_test,
                                   cmap=plt.cm.Blues,
                                   normalize=normalize)
      disp.ax_.set_title(title)
  
      print(title)
      print(disp.confusion_matrix)
  
  plt.show()
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/Edx9g.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Edx9g.png" alt="Confusion matrix"></a></p>