title: Barplot returns unsupported operand type(s) for -: 'str' and 'float'
body: <p>I try to plot a bar plot of a pivot_table with the following code:</p>
  
  <pre><code>fig=plt.figure(figsize=(10,10))
  ax1=fig.add_subplot(111)
  
  ax1.bar(Products.index,Products["Amount"].values)
  </code></pre>
  
  <p>The pivot table is like:</p>
  
  <pre><code>                                      Amount
  Material                             
  454T006S R 167 134:11                0.000000
  3457T006S R IE216 167 246:1          4.500000
  67T009S R 1510K304:1              36.000000
  7676009S R IE216 1510K304:1        30.500000
  676T012S 167 246:1                  1.000000
  2324T012S R 1510 111                6.000000
  1516T012S R 1510 151               50.500000
  1516T012S R 1510 20:1               26.500000
  </code></pre>
  
  <p>But it returns:</p>
  
  <blockquote>
    <p>unsupported operand type(s) for -: 'str' and 'float'</p>
  </blockquote>
  
  <p>What am I doing wrong?</p>