title: adding hatches to seaborn heatmap plot
body: <p>Is there a way to hatch particular 'cells' in a seaborn heatmap, which e.g. fullfill a condition?
  I already tried it with masked arrays and matplotlib pcolor, but it turned out that it hatched the wrong cells.</p>
  
  <pre><code>import numpy as np
  import seaborn as sns
  import matplotlib.pyplot as plt
  
  flights = sns.load_dataset("flights")
  flights = flights.pivot("month", "year", "passengers")
  zm = np.ma.masked_less(flights.values, 200)
  x= np.arange(0,12)
  y= np.arange(0,12)
  sns.heatmap(flights,linewidth=.1)
  plt.pcolor(x, y, zm, hatch='//', alpha=0.)
  
  plt.show()
  </code></pre>