title: Seaborn tsplot shows nothing
body: <p>I have a data frame called <code>amounts_month</code> of such a type:</p>
  
  <pre><code>    product accounting_month    amount
  0   A       201404              204748.0
  1   A       201405              445064.0
  2   B       201404              649326.0
  3   B       201405              876738.0
  4   C       201404              1046336.0
  </code></pre>
  
  <p>But when I evaluate</p>
  
  <pre><code>import seaborn as sns
  import matplotlib.pyplot as plt
  
  sns.tsplot(data=amounts_month,
            time='accounting_month',
            value='amount',
             condition='product'
            )
  </code></pre>
  
  <p>I get an empty plot. What's wrong with my code?</p>