title: Pandas - merge multiple time columns and fill values
body: <p>I have a dataframe (<code>df</code>):</p>
  
  <pre><code>index    time    val1    time2    val2
  0        0.2     1       0.1      2
  1        0.3     3       0.4      2
  2        0.4     1       0.8      9
  3                        1.0      2
  </code></pre>
  
  <p>I need to merge the <code>time</code> columns, sort them, then fill the missing values in a "downwards" manner.
  The expected output is like so:</p>
  
  <pre><code>index    time    val1    val2
  0        0.1     -       2
  1        0.2     1       2
  2        0.3     3       2
  3        0.4     1       2
  4        0.8     1       9
  5        1.0     1       2
  </code></pre>