title: Cannot interpolate dataframe even if most of the data is filled
body: <p>I tried to interpolate the NaN in my DataFrame using <code>interpolate()</code> method. However, the method failed with error : </p>
  
  <blockquote>
    <p>Cannot interpolate with all NaNs.</p>
  </blockquote>
  
  <p>Here's the code:</p>
  
  <pre><code>try:
      df3.interpolate(method='index', inplace=True)
      processor._arma(df3['TCA'])
  except Exception, e:
      sys.stderr.write('%s: [%s] %s\n' % (time.strftime("%Y-%m-%d %H:%M:%S"), nid3, e))
      sys.stderr.write('%s: [%s] len=%d\n' % (time.strftime("%Y-%m-%d %H:%M:%S"), nid3, len(df3.index)))
      sys.stderr.write('%s: [%s] %s\n' % (time.strftime("%Y-%m-%d %H:%M:%S"), nid3, df3.to_string()))
  </code></pre>
  
  <p>This is strange, because most of the data is already filled, as you can see in <strong>log 1</strong> or <strong>log 2</strong>. The length of the dataframe is 20, as all the data shown below. Even each cell is filled, I still can't use interpolate method. BTW, <code>df3</code> is a <strong>global</strong> value, I'm not sure if it would be a problem.</p>
  
  <hr>
  
  <p><strong>log 1</strong></p>
  
  <pre><code>2016-01-21 22:06:11: [ESIG_node_003_400585511] Cannot interpolate with all NaNs.
  2016-01-21 22:06:11: [ESIG_node_003_400585511] len=20
  2016-01-21 22:06:11: [ESIG_node_003_400585511]
                       TCA TCB TCC
  2016-01-21 20:06:22  19  17  18
  2016-01-21 20:06:23  19  17  18
  2016-01-21 20:06:24  18  18  18
  2016-01-21 20:06:25  18  17  18
  2016-01-21 20:06:26  18  18  18
  2016-01-21 20:06:27  19  18  18
  2016-01-21 20:06:28  19  17  18
  2016-01-21 20:06:29  18  18  18
  2016-01-21 20:06:30  18  17  18
  2016-01-21 20:06:31  19  17  18
  2016-01-21 20:06:32  18  17  18
  2016-01-21 20:06:33  18  18  18
  2016-01-21 20:06:34  19  18  18
  2016-01-21 20:06:35  18  17  18
  2016-01-21 20:06:36  19  18  18
  2016-01-21 20:06:37  18  18  18
  2016-01-21 20:06:38  18  18  18
  2016-01-21 20:06:39  19  18  18
  2016-01-21 20:06:40  18  17  18
  2016-01-21 20:06:41  18  18  18
  </code></pre>
  
  <p><strong>log 2</strong></p>
  
  <pre><code>2016-01-21 22:06:14: [ESIG_node_003_400585511] Cannot interpolate with all NaNs.
  2016-01-21 22:06:14: [ESIG_node_003_400585511] len=20
  2016-01-21 22:06:14: [ESIG_node_003_400585511]
                        TCA  TCB  TCC
  2016-01-21 20:06:33   18   18   18
  2016-01-21 20:06:34   19   18   18
  2016-01-21 20:06:35   18   17   18
  2016-01-21 20:06:36   19   18   18
  2016-01-21 20:06:37   18   18   18
  2016-01-21 20:06:38   18   18   18
  2016-01-21 20:06:39   19   18   18
  2016-01-21 20:06:40   18   17   18
  2016-01-21 20:06:41   18   18   18
  2016-01-21 20:06:42  NaN  NaN  NaN
  2016-01-21 20:06:43  NaN  NaN  NaN
  2016-01-21 20:06:44  NaN  NaN  NaN
  2016-01-21 20:06:45  NaN  NaN  NaN
  2016-01-21 20:06:46   19   18   18
  2016-01-21 20:06:47   18   17   18
  2016-01-21 20:06:48   18   18   18
  2016-01-21 20:06:49   19   18   18
  2016-01-21 20:06:50   18   17   18
  2016-01-21 20:06:51   18   18   18
  2016-01-21 20:06:52   19   17   18
  </code></pre>