title: pandas interpolate barycentric backward
body: <p>I have series where the first data can be a NaN value. 
  I tried interpolate( 'barycentric', limit_direction='both') but it does work if the first data is NaN:</p>
  
  <pre><code>pd.Series([ np.NaN, 1.5, 2]).interpolate( 'barycentric', limit_direction='both') 
  
  0    NaN
  1    1.5
  2    2.0
  dtype: float64
  </code></pre>
  
  <p>Is there a simple way to make it guess that the first number should be '1' ? Or is there a reason why it doesn't do it ? Other methods and directions don't seem to work. </p>