new_contents: from __future__ import absolute_import, division, print_function
  
  import pandas as pd
  
  from plotnine import ggplot, aes, stat_ecdf
  
  
  df = pd.DataFrame({'x': range(10)})
  
  
  def test_ecdf():
      p = ggplot(df, aes('x')) + stat_ecdf(size=2)
  
      assert p == 'ecdf'
  
subject: Add stat_ecdf tests