title: Log-log lmplot with seaborn
body: <p>Can the function <code>lmplot</code> from Seaborn plot on a log-log scale?
  This is lmplot on a normal scale</p>
  
  <pre><code>import numpy as np
  import pandas as pd
  import seaborn as sns
  x =  10**arange(1, 10)
  y = 10** arange(1,10)*2
  df1 = pd.DataFrame( data=y, index=x )
  df2 = pd.DataFrame(data = {'x': x, 'y': y}) 
  sns.lmplot('x', 'y', df2)
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/FXbtg.png" alt="sns.lmplot(&#39;x&#39;, &#39;y&#39;, df2)"></p>