title: Plot pandas data frame with year over year data
body: <p>I have a data frame in the format</p>
  
  <pre><code>              value
  2000-01-01    1
  2000-03-01    2
  2000-06-01    15
  2000-09-01    3
  2000-12-01    7
  2001-01-01    1
  2001-03-01    3
  2001-06-01    8
  2001-09-01    5
  2001-12-01    3
  2002-01-01    1
  2002-03-01    1
  2002-06-01    8
  2002-09-01    5
  2002-12-01    19
  </code></pre>
  
  <p>(index is datetime) and I need to plot all results year over year to compare the results each 3 months (The data can be monthly, too), plus the average of all years. </p>
  
  <p>I can easily plot they separately, but because of the index, it will shift the plots according with the index:</p>
  
  <pre><code>fig, axes = plt.subplots()
  df['2000'].plot(ax=axes, label='2000')
  df['2001'].plot(ax=axes, label='2001')
  df['2002'].plot(ax=axes, label='2002')
  axes.plot(df["2000":'2002'].groupby(df["2000":'2002'].index.month).mean())
  </code></pre>
  
  <p>So it's not the desired result. I've seem some answers here, but you have to concat, create a multiindex and plot. If one of the data frames has NaNs or missing values, it can be very cumbersome. Is there a pandas way to do it?</p>