title: Add a legend to my heatmap plot
body: <p>I'm trying to translate datas from csv file to a heatmap. For the moment, my heatmap looks like this : 
  <a href="https://i.stack.imgur.com/aPtpL.png" rel="noreferrer"><img src="https://i.stack.imgur.com/aPtpL.png" alt="enter image description here"></a></p>
  
  <p>But it miss a legend! This is my code :</p>
  
  <pre><code>import matplotlib.pyplot as plt
  import numpy as np
  
  column_labels = list(range(0,24))
  row_labels = ["Lundi",
                "Mardi",
                "Mercredi",
                "Jeudi",
                "Vendredi",
                "Samedi",
                "Dimanche"]
  data = np.array([
                  [0,0,0,0,0,0,0,0,0,0,434,560,650,340,980,880,434,434,0,0,0,0,0,0],
                  [0,0,0,0,0,0,0,434,560,0,650,0,0,0,0,340,980,0,0,0,880,0,434,343],
                  [0,0,0,0,0,0,0,0,0,0,434,560,650,340,980,880,434,434,0,0,0,0,0,0],
                  [0,0,0,0,0,0,0,434,560,0,650,0,0,0,0,340,980,0,0,0,880,0,434,343],
                  [0,0,0,0,0,0,0,0,0,0,434,560,650,340,980,880,434,434,0,0,0,0,0,0],
                  [0,0,0,0,0,0,0,434,560,0,650,0,0,0,0,340,980,0,0,0,880,0,434,343],
                  [0,0,0,0,0,0,0,0,0,0,434,560,650,340,980,880,434,434,0,0,0,0,0,0]
                  ])
  fig, axis = plt.subplots() # il me semble que c'est une bonne habitude de faire supbplots
  heatmap = axis.pcolor(data, cmap=plt.cm.Blues) # heatmap contient les valeurs
  
  axis.set_yticks(np.arange(data.shape[0])+0.5, minor=False)
  axis.set_xticks(np.arange(data.shape[1])+0.5, minor=False)
  
  axis.invert_yaxis()
  
  axis.set_yticklabels(row_labels, minor=False)
  axis.set_xticklabels(column_labels, minor=False)
  
  fig.set_size_inches(11.03, 3.5)
  
  plt.savefig('test.png', dpi=100)
  </code></pre>
  
  <p>How can I have a legend to this plot ? Like in this one :
  <a href="https://i.stack.imgur.com/HzVAv.png" rel="noreferrer"><img src="https://i.stack.imgur.com/HzVAv.png" alt="Heatmap made in javascript"></a></p>
  
  <p>Thanks a lot for your help !</p>