title: Pandas: How to format legend in MultiIndex
body: <p>I'm working at the moment on a plot with a MultiIndex Column. When I am trying to plot such data (as in the example below) in a diagram, I receive "(Semester1,Maths)","(Semester1,Science)" and so on in the legend.</p>
  
  <pre><code>import pandas as pd
  import numpy as np
  
  header = pd.MultiIndex.from_product([['Semester1','Semester2'],['Maths','Science']])
  d=([[12,45,67,56],[78,89,45,67],[45,67,89,90],[67,44,56,55]])
  
  
  df = pd.DataFrame(d,
                    index=['Alisa','Bobby','Cathrine','Jack'],
                    columns=header)
  df.plot(kind='bar')
  </code></pre>
  
  <p>I know that it is possible to set the legends text by hand, but is there also a way to use the Columns as a kind of variable, so that I can for example automatically adjust the legend to the format "Semester1, Maths" instead of "(Semester 1,Maths)"?</p>
  
  <p>Thanks for your support!</p>