title: Access pandas pivot table by row?
body: <p>We have a pandas dataframe with data borrowed from here:
  <a href="https://github.com/birdsarah/pydata-nc/tree/master/tutorial/assets" rel="nofollow noreferrer">Gapminder, country and income by year</a></p>
  
  <p>We use <code>dropna()</code> and <code>pivot_table()</code> and a mask <code>countries['Year'] &gt;= 1990</code> to create a new table that looks like this:</p>
  
  <pre><code>Country   Afghanistan      Albania     ...            Zambia     Zimbabwe
  Year                                   ...                               
  1990     11738.881323   754.357011     ...       3383.108434  4140.904818
  1991     12514.064579  1063.108731     ...       3561.065162  4133.270353
  1992     14607.470776  1126.386166     ...       3806.918331  4758.081606
  1993     18301.692593  1005.555801     ...       3740.561889  4911.218150
  1994     21889.609655   908.485392     ...       4298.488561  4672.967020
  
  
  Int64Index([1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000,
          2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011,
          2012, 2013, 2014, 2015],
         dtype='int64', name='Year')
  </code></pre>
  
  <p>Trying to do df[1990] returns a key error. What is the proper way to do it?</p>