title: vertical line in histogram with pyplot
body: <p>i've computed an Otsu's thresholding for a kinect depth image and now i want point out the optimal thresholding value on the histogram, using for example axvline with pyplot in opencv2. 
  I'm a beginner with python and programming too, this is the specific part of my code:</p>
  
  <pre><code>fig, (ax1, ax2, ax3) = plt.subplots(1, 3, figsize=(8, 2.5))
  ax1.imshow(img)
  ax1.set_title('Original')
  ax1.axis('off')
  
  ax2.hist(img)
  ax2.set_title('Histogram')
  plt.axvline(x=thresh, color='r', linestyle='dashed', linewidth=2)
  
  ax3.imshow(binary, cmap=plt.cm.gray)
  ax3.set_title('Thresholded')
  ax3.axis('off')
  
  plt.show()
  </code></pre>
  
  <p>but i don't know why, i obtain the vertical line on the thresholded plot</p>
  
  <p>what am i doing wrong??
  thanks</p>