title: qqplot not working with different sample sizes
body: <p>I'm trying to get started with the statsmodel package to make qqplots. I installed from source using the master branch with python 3.6. For what I'd like to do I want to make a qqplot comparing two data distributions of different sample sizes. I'm trying to just run the example code they have in the documentation, but it's throwing an error about the different sample sizes.</p>
  
  <pre><code>import numpy as np
  import matplotlib.pyplot as plt
  %matplotlib inline
  import statsmodels.api as sm
  from statsmodels.graphics.gofplots import qqplot
  
  # example 6
  x = np.random.normal(loc=8.25, scale=2.75, size=37)
  y = np.random.normal(loc=8.75, scale=3.25, size=57)
  pp_x = sm.ProbPlot(x, fit=True)
  pp_y = sm.ProbPlot(y, fit=True)
  fig = pp_x.qqplot(line='45', other=pp_y)
  title = 'Ex. 6 - qqplot - compare different sample sizes'
  h = plt.title(title)
  plt.show()
  </code></pre>
  
  <p>I get this error:</p>
  
  <blockquote>
    <p>ValueError: x and y must have same first dimension, but have shapes
    (57,) and (37,)</p>
  </blockquote>
  
  <p>Has anyone gotten this feature to work?</p>