title: Overleaped heatmap doesn't show correctly
body: <p>I have a heatmap plotted above an image (as shown on image link <a href="https://imgur.com/2vX6Bw6" rel="nofollow noreferrer">1</a>), with gaussian filter and normalize data. The main issue is that there is no value under 92 on the y axis, so the plot doesnt start on (0,0), instead start on (0,92). So, when I put together both pictures (heatmap and background image), there is an abrupt cut on the graph (as shown on 2nd link, where is the heatmap without background).</p>
  <p>So, how can I extend the axis on the heatmap so it start on (0,0)?</p>
  <p>Below is the code that I'm currently using to plot both images. Thanks!</p>
  <pre><code>import matplotlib.pyplot as plt
  import matplotlib.image as mpimg
  from scipy.ndimage.filters import gaussian_filter
  import matplotlib.colors as mcolors
  from sklearn.preprocessing import normalize
  
  x = df['x_data']
  y = df['y_data']
  
  heatmap, xedges, yedges = np.histogram2d(x, y, bins = [800,600])
  
  extent = [0, xedges[-1], yedges[0], yedges[-1]]
  heatmap = normalize(heatmap)
  heatmap = gaussian_filter(heatmap, 16)   
  
  colors = [(1,1-c,0,c) for c in np.linspace(0,1,100)]
  cmapred = mcolors.LinearSegmentedColormap.from_list('mycmap', colors, N=5)
  
  map_img = mpimg.imread('dir/to/background/image.png')
  
  fig, ax = plt.subplots(figsize=(16.1, 9.1))
  
  plt.imshow(map_img, extent=[0, 800, 0, 600], cmap = 'Greys_r')
  plt.imshow(heatmap.T, extent = extent, origin = 'lower', cmap = cmapred, alpha = 0.7)
  
  plt.ylim([0,600])
  plt.xlim([0,800])
  plt.show()
  </code></pre>
  <p><a href="https://imgur.com/2vX6Bw6" rel="nofollow noreferrer">Image of heatmap + background</a>: <a href="https://imgur.com/2vX6Bw6" rel="nofollow noreferrer">https://imgur.com/2vX6Bw6</a></p>
  <p><a href="https://imgur.com/axMe7K7" rel="nofollow noreferrer">Image of only heatmap</a>: <a href="https://imgur.com/axMe7K7" rel="nofollow noreferrer">https://imgur.com/axMe7K7</a></p>