title: Select column from GroupBy for boxplot
body: <p>I have a dataset I'd like to group by a certain field and then draw one or more boxplots of a single column. I am trying to do the grouping once then select each column in turn, but boxplot seems to draw all columns regardless.</p>
  
  <p>Here's an example:</p>
  
  <pre><code>import pandas as pd
  import matplotlib.pyplot as plt
  
  animals = pd.DataFrame({'kind': ['cat', 'dog', 'cat', 'dog', 'cat', 'dog'],
                          'height': [9.1, 6.0, 9.5, 34.0, 8.0, 30.0],
                          'weight': [7.9, 7.5, 9.9, 198.0, 8.0, 150.0]})
  </code></pre>
  
  <p>Grouping by 'kind' and then boxplot gives me four boxes (cat, height) (cat, weight) (dog,height) (dog,weight) as expected.</p>
  
  <pre><code>grouped = animals.groupby('kind')
  grouped.boxplot(subplots=False)
  </code></pre>
  
  <p>Now if I just want the height I thought this would work:</p>
  
  <pre><code>grouped[["height"]].boxplot(subplots=False)
  </code></pre>
  
  <p>but I just get the same four boxes.</p>
  
  <p>I know I can do </p>
  
  <pre><code>animals.boxplot(column="height", by="kind")
  </code></pre>
  
  <p>and this gives me what I want, but this seems inefficient; if I have 10 columns I am doing the grouping ten times.</p>
  
  <p>To clarify, I want something like</p>
  
  <pre><code>grouped = mydata.groupby('something')
  for col_name in [...column_names...]:
    grouped[[col_name]].boxplot(...)
    ...save plot to file
  </code></pre>
  
  <p>i.e. one grouping operation and many boxplots.</p>
  
  <p>What is the correct way to select columns out of a <code>DataFrameGroupBy</code>?</p>
  
  <p>Using pandas 0.25.1 and Python 3.7.3</p>