title: Pandas: multiple bar plot from aggregated columns
body: <p>In python pandas I have create a dataframe with one value for each year and two subclasses - i.e., one metric for a parameter triplet</p>
  
  <pre><code>import pandas, requests, numpy
  import matplotlib.pyplot as plt
  
  df
  
         Metric    Tag_1  Tag_2  year
  0     5770832  FOOBAR1  name1  2008
  1     7526436  FOOBAR1    xyz  2008
  2    33972652  FOOBAR1  name1  2009
  3    17491416  FOOBAR1    xyz  2009
  ...
  16    6602920  baznar2  name1  2008
  17       6608  baznar2    xyz  2008
  ...
  30  142102944  baznar2  name1  2015
  31          0  baznar2    xyz  2015
  </code></pre>
  
  <p>I would like to produce a bar plot with metrics as y-values over x=(year,Tag_1,Tag_2) and sorting primarily for years and secondly for tag_1 and color the bars depending on tag_1. Something like</p>
  
  <pre><code>(2008,FOOBAR,name1)   --&gt; 5770832  *RED*
  (2008,baznar2,name1)  --&gt; 6602920  *BLUE*
  (2008,FOOBAR,xyz)     --&gt; 7526436  *RED*
  (2008,baznar2,xyz)    --&gt; ...      *BLUE*
  (2008,FOOBAR,name1)   --&gt; ...      *RED*
  </code></pre>
  
  <p>I tried starting with a grouping of columns like</p>
  
  <pre><code>df.plot.bar(x=['year','tag_1','tag_2']
  </code></pre>
  
  <p>but have not found a way to separate selections into two bar sets next to each other.</p>