title: why my scatter plot shows no color?
body: <p>I am working on scattering plot using matplotlib and just simply testing 
    several sample codes, but cannot show colors. Only showing gray.
    For example:</p>
  
  <pre><code>import numpy as np
  import matplotlib.pyplot as plt
  
  x = np.random.rand(100)
  y = np.random.rand(100)
  t = np.arange(100)
  
  plt.scatter(x, y, c=t)
  plt.show()
  </code></pre>
  
  <p>This is the exactly same as <a href="https://stackoverflow.com/questions/17682216/scatter-plot-and-color-mapping-in-python">this previous question about scatterplots and color mapping</a>. See the execution result:</p>
  
  <p><img src="https://i.stack.imgur.com/MGil9.png" alt="Screenshot of results"></p>
  
  <p>What's wrong with my setup?
     I am using Jupyter python 3.x version.   </p>
  
  <p>Thanks!</p>