title: Pandas: how to draw a bar plot with two categories and four series each?
body: <p>I have the following dataframe, where <code>pd.concat</code> has been used to group the columns:</p>
  
  <pre><code>    a               b            
     C1  C2  C3  C4  C5  C6  C7  C8
  0  15  37  17  10   8  11  19  86
  1  39  84  11   5   5  13   9  11
  2  10  20  30  51  74  62  56  58
  3  88   2   1   3   9   6   0  17
  4  17  17  32  24  91  45  63  48
  </code></pre>
  
  <p>Now I want to draw a bar plot where I only have two categories (<code>a</code> and <code>b</code>), and each category has four bars representing the average of each column. Columns C1 and C5 should have the same color, as should columns C2 and C6, and so forth. </p>
  
  <p><strong>How can I do it with <em>df.plot.bar()?</em></strong></p>
  
  <p>The plot should resemble the following image. Sorry for it being hand-drawn but it was very hard for me to find a relevant example:
  <a href="https://i.stack.imgur.com/8I1Z6.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/8I1Z6.jpg" alt="enter image description here"></a></p>
  
  <p><strong>EDIT</strong></p>
  
  <p>This is the header of my actual DataFrame:</p>
  
  <pre><code>    C1  C2  C3  C4  C5  C6  C7  C8
  0   34  34  34  34  6   40  13  26
  1   19  19  19  19  5   27  12  15
  2   100 100 100 100 0   0   0   0
  3   0   0   0   0   0   0   0   0
  4   100 100 100 100 0   0   0   0
  </code></pre>