title: Lines to separate groups in seaborn heatmap
body: <p>I am plotting data as a Seaborn heatmap in Python. My data is intrinsically grouped into categories, and I'd like to have lines on the plot to indicate where the groups lie on the map. As a simple example, suppose I wanted to modify this plot from the documentation...</p>
  
  <pre><code>import seaborn as sns; sns.set()
  flights = sns.load_dataset("flights")
  flights = flights.pivot("month", "year", "passengers")
  ax = sns.heatmap(flights, cbar=False)
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/RELwf.png" rel="noreferrer"><img src="https://i.stack.imgur.com/RELwf.png" alt="enter image description here"></a></p>
  
  <p>Where I wanted to emphasize the comparisons between quarters of the year by making a plot like the one below; how would I do that?</p>
  
  <p><a href="https://i.stack.imgur.com/lO3uP.png" rel="noreferrer"><img src="https://i.stack.imgur.com/lO3uP.png" alt="enter image description here"></a></p>