title: Pandas Interpolation: {ValueError}Invalid fill method. Expecting pad (ffill) or backfill (bfill). Got linear
body: <p>I am trying to interpolate time series data, <code>df</code>, which looks like:</p>
  
  <pre><code>         id      data        lat      notes    analysis_date
  0  17358709       NaN  26.125979      None     2019-09-20 12:00:00+00:00
  1  17358709       NaN  26.125979      None     2019-09-20 12:00:00+00:00
  2  17352742 -2.331365  26.125979      None     2019-09-20 12:00:00+00:00
  3  17358709 -4.424366  26.125979      None     2019-09-20 12:00:00+00:00
  </code></pre>
  
  <p>I try: <code>df.groupby(['lat', 'lon']).apply(lambda group: group.interpolate(method='linear'))</code>, and it throws <code>{ValueError}Invalid fill method. Expecting pad (ffill) or backfill (bfill). Got linear</code>
  I suspect the issue is with the fact that I have <code>None</code> values, and I do not want to interpolate those. What is the solution?</p>
  
  <p><code>df.dtypes</code> gives me:</p>
  
  <pre><code>id                                                                int64
  data                                                            float64
  lat                                                             float64
  notes                                                            object
  analysis_date         datetime64[ns, psycopg2.tz.FixedOffsetTimezone...
  dtype: object
  </code></pre>