title: How can you interpolate NaN values if first few rows have NaN values?
body: <p><strong>Description:</strong>
  I am trying to interpolate missine values (represented as NaN), however the method is only work on the NaN values between known values. I am slo quite confused on how the value of the missing values are computed in bfill. As I understand it is only fill missing values by the same value as the first succeeding known value. Here's an example:</p>
  
  <pre><code>&gt;&gt;&gt; df = pd.DataFrame([['M', '2014-01-01 00:26:00', '2'], ['M', 'M', 'M'], ['M', '2014-01-01 00:26:30', 9],[5, '2014-01-01 00:26:50', 'M'],[6, '2014-01-01 00:26:50', 'M']], columns=['x','y','z'])
  &gt;&gt;&gt; df
     x                    y  z
  0  M  2014-01-01 00:26:00  2
  1  M                    M  M
  2  M  2014-01-01 00:26:30  9
  3  5  2014-01-01 00:26:50  M
  4  6  2014-01-01 00:26:50  M
  &gt;&gt;&gt; df = df.replace(['M'],[np.NaN])
  &gt;&gt;&gt; df
      x                    y    z
  0 NaN  2014-01-01 00:26:00    2
  1 NaN                  NaN  NaN
  2 NaN  2014-01-01 00:26:30    9
  3   5  2014-01-01 00:26:50  NaN
  4   6  2014-01-01 00:26:50  NaN
  &gt;&gt;&gt; df['x'] = df['x'].astype(np.float64)
  &gt;&gt;&gt; df['z'] = df['z'].astype(np.float64)
  &gt;&gt;&gt; df['y'] = pd.to_datetime(df['y'])
  &gt;&gt;&gt; df
      x                   y   z
  0 NaN 2014-01-01 00:26:00   2
  1 NaN                 NaT NaN
  2 NaN 2014-01-01 00:26:30   9
  3   5 2014-01-01 00:26:50 NaN
  4   6 2014-01-01 00:26:50 NaN
  &gt;&gt;&gt; df.interpolate()
      x                   y    z
  0 NaN 2014-01-01 00:26:00  2.0
  1 NaN                 NaT  5.5
  2 NaN 2014-01-01 00:26:30  9.0
  3   5 2014-01-01 00:26:50  9.0
  4   6 2014-01-01 00:26:50  9.0
  &gt;&gt;&gt; df.interpolate(method='bfill')# try to fill first three rows in x
      x                   y   z
  0   2 2014-01-01 00:26:00   2
  1 NaN                 NaT NaN
  2   9 2014-01-01 00:26:30   9
  3   5 2014-01-01 00:26:50 NaN
  4   6 2014-01-01 00:26:50 NaN
  </code></pre>
  
  <p><strong>Goal:</strong>
  I want to fill x and z and if there is a possibility to fill y which has datetime type.</p>