title: Seaborn Facet Grid: Adding horizontal Mean line to each plt.plot facet
body: <p>I have  a simple facet grid of 4 rows and 1 column with lineplots denoting different categories for the facets - image below.</p>
  
  <pre><code>#lineplot for each Category over the last three years
  
  g = sns.FacetGrid(dataframe, row="Category", sharey=False, sharex=False, height=2.5, aspect = 3)
  g = g.map(plt.plot, 'CREATED_DATE', 'COUNT')
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/XOc0u.png" alt="image"></p>
  
  <p>How do I add a reference line showing the mean Count for each facet?</p>