title: Plotting relative frequency of every unique element of a dataframe
body: <p>How do I get from this dataset (NaNs are strings):</p>
  <pre><code>         0       1       2
  0   item_1  item_2  item_3
  1   item_1  item_2     NaN
  2   item_2     NaN     NaN
  3   item_1  item_2     NaN
  4   item_2  item_3     NaN
  </code></pre>
  <p>Into this sorted relative frequency plot:</p>
  <p><a href="https://i.stack.imgur.com/F7n6A.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/F7n6A.png" alt="enter image description here" /></a></p>