title: matplotlib scatter: TypeError: unhashable type: 'numpy.ndarray'
body: <p>I'm getting the following error:</p>
  
  <pre><code>TypeError                                 Traceback (most recent call last)
  ~/.local/share/miniconda3/lib/python3.6/site-packages/matplotlib/colors.py in to_rgba(c, alpha)
      154     try:
  --&gt; 155         rgba = _colors_full_map.cache[c, alpha]
      156     except (KeyError, TypeError):  # Not in cache, or unhashable.
  
  TypeError: unhashable type: 'numpy.ndarray'
  </code></pre>
  
  <p>The code in question is from a <code>.ipynb</code> downloaded from Coursera.</p>
  
  <p>It works fine on their system, but it seems that I have a library versioning problem locally.</p>
  
  <p>The code is:</p>
  
  <pre><code>plt.scatter(X[0, :], X[1, :], c=y, cmap=plt.cm.Spectral)
  </code></pre>