title: Python ggplot is not plotting dataframe
body: <p>I am trying to plot the following data frame. X and Y are lists. After I prepare those lists I construct the data frame via dictionary. But when I plot it doesn't actually plot.</p>
  
  <pre><code>df = pd.DataFrame({"day": day, "ndcg@k": x, "MAP@k": y})
  df2 = pd.melt(df[['day', 'ndcg@k', 'MAP@k']], id_vars=['day'])
  ggplot(aes(x='day', y='value', group='variable', shape='variable', colour='variable'), data=df2) + geom_line() + geom_point()
  </code></pre>
  
  <p>And my data looks like this (both before melt and after melt)</p>
  
  <pre><code>      MAP@k       day    ndcg@k
  0  0.201919  20150203  0.245559
  1  0.198214  20150204  0.241085
  
          day variable     value
  0  20150203   ndcg@k  0.245559
  1  20150204   ndcg@k  0.241085
  2  20150203    MAP@k  0.201919
  3  20150204    MAP@k  0.198214
  </code></pre>