title: Pandas fillna with an incremented value
body: <p>I have a dataframe with a column of sequential but not adjacent numbers and missing values.</p>
  
  <p>I'd like to use the <code>fillna</code> function to fill in the missing values with an incremented value from the previous non-missing row.</p>
  
  <p>Here's a simplified table:</p>
  
  <pre><code>index  my_counter
  0      1
  1      2
  2      NaN
  3      3
  4      NaN
  5      NaN
  6      8
  </code></pre>
  
  <p>I'd like to fill in <code>my_counter</code> as such:</p>
  
  <pre><code>index  my_counter
  0      1
  1      2
  2      2.1
  3      3
  4      3.1
  5      3.2
  6      8
  </code></pre>
  
  <p>How can I accomplish this task?</p>