title: ValueError: Could not interpret input 'index' when using index with seaborn lineplot
body: <p>I want the use the index of a pandas DataFrame as x value for a seaborn plot. However, this raises a value error.</p>
  <p>A small test example:</p>
  <pre><code>import pandas as pd
  import seaborn as sns
  sns.lineplot(x='index',y='test',hue='test2',data=pd.DataFrame({'test':range(9),'test2':range(9)}))
  </code></pre>
  <p>It raises:</p>
  <pre><code>ValueError: Could not interpret input 'index'
  </code></pre>
  <p>Is it not possible to use the index as x values? What am I doing wrong?
  Python 2.7, seaborn 0.9</p>