title: Pandas groupby agg with multiple functions to apply returns error
body: <p><strong>Problem</strong> Attempting a groupby on a simple <a href="https://drive.google.com/open?id=0B6ymLohJIbhCQzNEdDdTaFVHSU0" rel="nofollow noreferrer">dataframe</a> (downloadable csv) and then agg to return aggregate values for columns (size, sum, mean, std deviation). What seems like a simple problem is giving an unexpectedly challenging error.</p>
  
  <pre><code>Top15.groupby('Continent')['Pop Est'].agg(np.mean, np.std...etc)
  # returns 
  ValueError: No axis named &lt;function std at 0x7f16841512f0&gt; for object type &lt;class 'pandas.core.series.Series'&gt;
  </code></pre>
  
  <p>What I am trying to get is a df with index set to continents and columns <code>['size', 'sum', 'mean', 'std']</code></p>
  
  <p><strong>Example Code</strong></p>
  
  <pre><code>import pandas as pd
  import numpy as np
  
  # Create df
  df = pd.DataFrame({'Country':['Australia','China','America','Germany'],'Pop Est':['123','234','345','456'],'Continent':['Asia','Asia','North America','Europe']})
  
  # group and agg
  df = df.groupby('Continent')['Pop Est'].agg('size','sum','np.mean','np.std')
  </code></pre>