title: Python 3d scatterplot colormap issue
body: <p>I have four dimensional data (x, y, z displacements; and respective voltages) which I wish to plot in a 3d scatterplot in python. I've gotten the 3d plot to render, but I want to have the colour of the points change using a colourmap, dependent upon the magnitude of the point's voltage. </p>
  
  <p>I've tried a few things, but can't seem to get it to work I'm getting the error <code>ValueError: Cannot convert argument type &lt;type 'numpy.ndarray'&gt; to rgba array</code>. I'm not sure exactly how to convert what I need to convert, so if anybody could please offer some help, I'd be most appreciative.</p>
  
  <p>My code is here:</p>
  
  <pre><code>fig = plt.figure()
  from mpl_toolkits.mplot3d import Axes3D
  cmhot = plt.cm.get_cmap("hot")
  ax = fig.add_subplot(111, projection='3d',)
  ax.scatter(x, y, z, v, s=50, c = cmhot)
  plt.show()
  </code></pre>