title: statsmodels.api and scipy.stats not producing proper fit
body: <p>I'm trying to plot a line of best fit through two sets of data with <code>scipy.stats</code> and <code>statsmodels.api</code>.</p>
  
  <pre><code>import matplotlib.pyplot as plt
  import numpy as np
  import statsmodels.api as sm
  from scipy import stats
  
  # toy data
  y1 =  np.array([1,2,3,4,5])
  x1 =  np.array([2,4,6,8,10])
  y2 = np.array([1,3.0,5.0,7.0,9.0])
  x2 = np.array([1,2.9,5.3,7.4,8.9])
  
  #  should produce straight lines through each data set
  plt.scatter(x1, y1, label = 'LRIS')
  plt.scatter(x2, y2, label = 'PFCam')
  for x, y in zip([x1, x2], [y1, y2]):
      model = sm.OLS(y, sm.add_constant(x))
      results = model.fit()
      params = stats.linregress(x, y)
      plt.plot(params[0]*x + params[1])
  
  plt.xlabel('log Integration time, t [s]')
  plt.ylabel('V [mag]')
  plt.legend()
  plt.show()
  </code></pre>
  
  <p>produces</p>
  
  <p><a href="https://i.stack.imgur.com/YBxmx.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/YBxmx.png" alt="enter image description here"></a></p>
  
  <p>I don't understand what's going on to produce the lines of 'best' fit like this.</p>