title: Python Matplotlib - Misaligned Grid Lines and Color Fills
body: <p>I'm using the following code to produce a sort of binary heatmap:</p>
  
  <pre><code>import numpy as np
  import matplotlib.colors as mlc
  import matplotlib.pyplot as mlp
  
  states = ['AAAA', 'BBBB', 'CCCC', 'DDDD']
  walk = [1, 2, 3, 2, 3, 2, 3, 2, 3, 2]
  
  states_len = len(states)
  walk_len = len(walk)
  
  img = np.zeros((states_len, walk_len), dtype=float)
  
  for i, s in enumerate(walk):
      img[s, i] = 1.0
  
  figure, ax = mlp.subplots()
  
  color_map = mlc.LinearSegmentedColormap.from_list('ColorMap', [(1.000, 1.000, 1.000), (0.984, 0.501, 0.447)])
  ax.imshow(img, cmap=color_map, interpolation='none')
  
  ax.set_xlabel('Steps', fontsize=13)
  ax.set_xticks(np.arange(0, walk_len, 1))
  ax.set_xticks(np.arange(-0.5, walk_len, 1), minor=True)
  ax.set_xticklabels(np.arange(1, walk_len + 1, 1))
  
  ax.set_ylabel('States', fontsize=13)
  ax.set_yticks(np.arange(0, states_len, 1))
  ax.set_yticks(np.arange(-.5, states_len, 1), minor=True)
  ax.set_yticklabels(states)
  
  ax.grid(which='minor', color='k')
  
  ax.set_title('Walkplot (Sequence)', fontsize=15, fontweight='bold')
  </code></pre>
  
  <p>And it's producing weird results:</p>
  
  <p><a href="https://i.stack.imgur.com/pMLCP.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/pMLCP.png" alt="Screenshot"></a></p>
  
  <p>As you may notice, the color spots look not lined up with respect to the grid squares. On the top of that, sometimes grid lines don't have a uniform size, some looks bigger than others. </p>
  
  <p>I think it must be related to the aspect ratio or to the figure size, but I'm clueless about a potential fix. How can I solve this issue?</p>
  
  <p>My current setup:</p>
  
  <ul>
  <li>Python 3.6 x64</li>
  <li>PyCharm 2018.1</li>
  </ul>