title: interpolate previous day values using pandas
body: <p>I use <code>ffill</code> to interpolate previous values but what if the whole day missing values in column b but not in column a, can I fill them with the previous day values by keeping the same structure like shown below?</p>
  <p>input</p>
  <pre><code>datetime  a b
  2020-01-01 00:00:00  10 20
  2020-01-01 01:00:00  15 30
  2020-01-01 03:00:00  16 40
  2020-01-01 04:00:00  17 50
  2020-01-01 05:00:00  15 60
  2020-01-02 00:00:00  10 0
  2020-01-02 01:00:00  15 0
  2020-01-02 03:00:00  16 0
  2020-01-02 04:00:00  17 0
  2020-01-02 05:00:00  15 0
  2020-01-03 00:00:00  10 10
  2020-01-03 01:00:00  15 0
  2020-01-03 03:00:00  16 0
  2020-01-03 04:00:00  17 0
  2020-01-03 05:00:00  15 0
  </code></pre>
  <p>output</p>
  <pre><code>datetime  a b
  2020-01-01 00:00:00  10 20
  2020-01-01 01:00:00  15 30
  2020-01-01 03:00:00  16 40
  2020-01-01 04:00:00  17 50
  2020-01-01 05:00:00  15 60
  2020-01-02 00:00:00  10 20
  2020-01-02 01:00:00  15 30
  2020-01-02 03:00:00  16 40
  2020-01-02 04:00:00  17 50
  2020-01-02 05:00:00  15 60
  2020-01-03 00:00:00  10 10
  2020-01-03 01:00:00  15 0
  2020-01-03 03:00:00  16 0
  2020-01-03 04:00:00  17 0
  2020-01-03 05:00:00  15 0
  </code></pre>