title: Why plt.imshow() doesn't display the image?
body: <p>I am a newbie to keras, and when I tried to run my first keras program on my linux, something just didn't go as I wish.
  Here is my python code:</p>
  
  <pre><code>import numpy as np
  np.random.seed(123)
  from keras.models import Sequential
  from keras.layers import Dense, Dropout, Activation, Flatten
  from keras.layers import Convolution2D, MaxPooling2D
  from keras.utils import np_utils
  from keras.datasets import mnist
  (X_train,y_train),(X_test,y_test) = mnist.load_data()
  print X_train.shape
  from matplotlib import pyplot as plt
  plt.imshow(X_train[0])
  </code></pre>
  
  <p>The last sentence doesn't display anything. I copied those codes from a tutorial with out any modification. And there is nothing wrong with the backend of matplotlib on my computer. I have tested that through the code below.</p>
  
  <pre><code>import matplotlib.pyplot as plt
  
  data = [[0, 0.25], [0.5, 0.75]]
  
  fig, ax = plt.subplots()
  im = ax.imshow(data, cmap=plt.get_cmap('hot'), interpolation='nearest',
                 vmin=0, vmax=1)
  fig.colorbar(im)
  plt.show()
  </code></pre>
  
  <p>And then I got a image like that:
  <a href="https://i.stack.imgur.com/2r6Lw.png" rel="noreferrer"><img src="https://i.stack.imgur.com/2r6Lw.png" alt="enter image description here"></a></p>
  
  <p><br>
  Moreover, I can get X_train[0] printed and it seems nothing wrong.<br>
  So what could be the reason for that? Why the imshow() function in my first code didn't display anything?</p>