title: How to change dataframe shape/format to turn column names as identifier column
body: <p>I have an input data frame that looks similar to this, where the value-entries are <strong>area</strong>:</p>
  <p><a href="https://i.stack.imgur.com/l1faF.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/l1faF.png" alt="input data frame format" /></a></p>
  <p>There are an arbitrary number of frames and columns and some of the columns 0, 1, ... will have NaN values for some frames.</p>
  <p>I would like to &quot;massage&quot; this it into a format like this:</p>
  <p><a href="https://i.stack.imgur.com/T10Hu.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/T10Hu.png" alt="target data frame format" /></a></p>
  <p>My goal is that I can then use e.g. <code>factorplot</code> from Seaborn to plot <em>area</em> vs. <em>frame</em> categorized by <em>cell_rank</em>.</p>