title: Plot on top of seaborn clustermap
body: <p>I generated a clustermap using <code>seaborn.clustermap</code>.
  I'd like to draw/plot an horizontal line on top of the heatmap like in this figure<a href="https://i.stack.imgur.com/zTdmc.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/zTdmc.png" alt="enter image description here"></a></p>
  
  <p>I simply tried to use matplotlib as:</p>
  
  <pre><code>plt.plot([x1, x2], [y1, y2], 'k-', lw = 10)
  </code></pre>
  
  <p>but the line is not displayed.
  The object returned by <code>seaborn.clustermap</code> doesn't have any properties like in this similar <a href="https://stackoverflow.com/questions/31749448/how-to-add-percentages-on-top-of-bars-in-seaborn">question</a>.
  How can I plot the line?</p>
  
  <p>Here is the code that generates a "random" clustermap similar to the one I posted:</p>
  
  <pre><code>import numpy as np
  import seaborn as sns
  import pandas as pd
  import matplotlib.pyplot as plt
  import random 
  
  data = np.random.random((50, 50))
  df = pd.DataFrame(data)
  row_colors = ["b" if random.random() &gt; 0.2 else "r"  for i in range (0,50)]
  cmap = sns.diverging_palette(133, 10, n=7, as_cmap=True)
  result = sns.clustermap(df, row_colors=row_colors, col_cluster = False, cmap=cmap, linewidths = 0)
  plt.plot([5, 30], [5, 5], 'k-', lw = 10)
  plt.show()
  </code></pre>