title: Grid is not correct with imshow matrix
body: <p>I am trying to make a confusion matrix plot, or just any plot with quadrants and something in it. I used imshow.</p>
  
  <pre><code>cm = [[370288,  5190],
         [ 2213,   6758]]
  
  with plt.style.context("seaborn-white"):
  
      ax = plt.gca()
      ax.imshow(cm, interpolation='none', vmin=1, vmax=1)
      classNames = ['no churn','churn']
      ax.set_title('Confusion Matrix',fontsize=20, fontweight='bold', color='indigo')
      ax.set_ylabel('actual',fontsize=15, fontweight='bold')
      ax.set_xlabel('predicted',fontsize=15, fontweight='bold')
      tick_marks = np.arange(len(classNames))
      plt.xticks(tick_marks, classNames)
      plt.yticks(tick_marks, classNames, rotation=90)
      plt.grid(color='indigo', linestyle='-', linewidth=5)
      s = [['TN','FP'], ['FN', 'TP']]
  
  
  
      for i in range(2):
          for j in range(2):
              plt.text(j,i, str(s[i][j])+" = "+annot[i][j], size=12, ha='center', va='center')
      plt.show()
  </code></pre>
  
  <p>Unfortunately I don't manage to make the grid right. If I add</p>
  
  <pre><code>ax.set_xticks(np.arange(-.5, 1.5, 1))
  ax.set_yticks(np.arange(-.5, 1.5, 1))
  </code></pre>
  
  <p>then the grid is correct but the x and y axis labels are not centered anymore and right and left border are not bold. How can I have bold borders and the correct grid that divides the whole plot into four quadrants?</p>