title: matplotlib 3d scatter plot date
body: <p>I have a list of dates in format <code>15/10/2017</code></p>
  
  <p>I have tried the following</p>
  
  <pre><code>from matplotlib import pyplot
  import pandas as pd
  
  dates = ['15/10/2016', '16/10/2016', "17/10/2015", "15/10/2014"]
  dates_formatted = [pd.to_datetime(d) for d in dates ]
  x = [1,2,3,4]
  z = [5,6,7,8]
  
  pyplot.scatter(x, dates_formatted, z)
  pyplot.show()
  </code></pre>
  
  <p>It throws an error <code>TypeError: ufunc 'sqrt' not supported for the input types, and the inputs could not be safely coerced to any supported types according to the casting rule ''safe''</code></p>
  
  <p>It shows if it is 2D. e.g. <code>pyplot.scatter(x, dates_formatted)</code></p>
  
  <p>I have also tried the following</p>
  
  <pre><code>ax = Axes3D(fig)
  ax = fig.add_subplot(111,projection='3d')
  ax.scatter(x, dates_formatted, y)
  pyplot.show()
  </code></pre>
  
  <p>It throws an error <code>Float() argument must be a string or number</code></p>