title: imshow(img, cmap=cm.gray) shows a white for 128 value
body: <p>I'm moving from MatLab to python and playing around with the imshow function.</p>
  
  <p>I can't seem to get my head around why it doesn't show the value 128 as grey with I have chosen the cmap to be gray-scale.</p>
  
  <p><img src="https://i.stack.imgur.com/AlqpQ.png" alt="Code example"></p>
  
  <p>It seems as it uses the grayscale for highest (128) and lowest values.. I want it to use the grayscale for [0:255]. How do I do that?</p>