title: multiple bars in python
body: <p>I want to plot multiple bars over each other, with matplotlib.
  I have used:</p>
  
  <pre><code>a=(45,22,17,28)
  
  b=(32,17,15,27)
  
  c=(15,18,22,25)
  
  rects1 = plt.bar(index, a, bar_width, alpha=opacity, color='b',error_kw=error_config,  label='A')
  
  rects2 = plt.bar(index, b, bar_width,alpha=opacity, color='r', error_kw=error_config,   label='B',bottom=a)
  
  rects4 = plt.bar(index , c, bar_width, alpha=opacity, color='y', error_kw=error_config, label='C',bottom=a+b)
  </code></pre>
  
  <p>I would like to have c over b over a, but bottom=a+b doesn't work...</p>