title: overlay two densities in one plot
body: <p>I have the below function which plots density for a_positions. Now, I would like add b_positions which overlay with a_positions with a trasparent colours. How is possible to overlay a_positions and b_positions in one plot with the below function?</p>
  
  <pre><code>def plot_density(a_positions, b_positions, chr_name, chr_len, out_fname):
      d = pd.DataFrame(a_positions)
      d.hist(bins=100)
      ax = plt.gca()
      ax.set_xlabel(chr_name + " positions")
      ax.set_ylabel("Density")
  
      ax.set_title('', color='black')
      #plt.subplots_adjust(bottom=.25, left=.25)
      plt.ticklabel_format(style='plain')
  
      ax.set_xlim(xmin=1, xmax=chr_len)
      ax.set_xticks(pd.np.linspace(1, chr_len, 5))
      plt.savefig(out_fname)
      plt.close()
  </code></pre>