title: python - matplotlib plt.plot parameter
body: <p>HI  i am python beginner</p>
  
  <p>I just practice scikit-learn this topic</p>
  
  <p><a href="http://scikit-learn.org/stable/auto_examples/feature_selection/plot_permutation_test_for_classification.html" rel="nofollow noreferrer">Test with permutations the significance of a classification score</a></p>
  
  <p>I was confused about this matplotlib plot method</p>
  
  <pre><code>plt.hist(permutation_scores, 20, label='Permutation scores',edgecolor='black')
  ylim = plt.ylim()
  plt.plot(2 * [score], ylim, '--g', linewidth=3,label='Classification Score'' (pvalue %s)' % pvalue)
  plt.plot(2 * [1. / n_classes], ylim, '--k', linewidth=3, label='Luck')
  plt.ylim(ylim)
  plt.legend()
  plt.xlabel('Score')
  plt.show()
  </code></pre>
  
  <p>here why use 2 * [score] rather then 2 * score</p>
  
  <p>if i use 2*score then figure cannot display correctly </p>
  
  <pre><code>plt.plot(2 * [score], ylim, '--g', linewidth=3,label='Classification Score'' (pvalue %s)' % pvalue)
  </code></pre>
  
  <p>2 * [score] = [0.51333333333333331, 0.51333333333333331]</p>
  
  <p>but if i draw a 2-d plot
  the x should be 1 number ?
  and i wrong?</p>