title: Fill na by applying condition on another column
body: <p>There is a df which contains two columns. First column has monthly values but the second one only contains quarterly values. I want to fill the NA values of second column by the same percentage change on the first column. For example, the original df looks like this:</p>
  <pre><code>            ColA   ColB 
  2019-12-31   100    5
  2020-01-31   200    NA
  2020-02-28   300    NA
  2020-03-31   200    20
  2020-04-30   300    NA
  </code></pre>
  <p>And I want to calculate the percentage change of colA and then fill the colB as per the percentage change. The result should look like:</p>
  <pre><code>            ColA   ColB 
  2019-12-31   100    5
  2020-01-31   200    10
  2020-02-28   300    15
  2020-03-31   200    20
  2020-04-30   300    30
  </code></pre>
  <p>I can calculate the percentage changes of colA by <code>pct_change()</code> but not getting the point how to apply only on the na values of colB. Is there any solution for it please?</p>