title: Polar plot - Put one grid line in bold
body: <p>I am trying to make use the polar plot projection to make a radar chart. I would like to know how to put only one grid line in bold (while the others should remain standard).</p>
  
  <p>For my specific case, I would like to highlight the gridline associated to the ytick "0". </p>
  
  <pre><code>from matplotlib import pyplot as plt
  import pandas as pd
  import numpy as np
  #Variables
  sespi = pd.read_csv("country_progress.csv")
  labels = sespi.country
  progress = sespi.progress
  angles=np.linspace(0, 2*np.pi, len(labels), endpoint=False)
  #Concatenation to close the plots
  progress=np.concatenate((progress,[progress[0]]))
  angles=np.concatenate((angles,[angles[0]]))
  
  #Polar plot
  fig=plt.figure()
  ax = fig.add_subplot(111, polar=True)
  ax.plot(angles, progress, '.--', linewidth=1, c="g")
  #ax.fill(angles, progress, alpha=0.25)
  ax.set_thetagrids(angles * 180/np.pi, labels)
  ax.set_yticklabels([-200,-150,-100,-50,0,50,100,150,200])
  #ax.set_title()
  ax.grid(True)
  plt.show()
  </code></pre>