title: Matplotlib automatic axis gives unexpected result
body: <p>I'm plotting some data with Matplotlib but the auto axis labels are off. The X axis has negative values in the label where they should be positive and the Y axis has a very strange scale. </p>
  
  <p>Here's some code: </p>
  
  <pre><code>import numpy as np
  import numpy.polynomial.polynomial as poly
  import matplotlib.pylab as plot
  
  # pass in same sized arrays x, y, and z, returns fit of x to z and y to z 
  def fitData(x, y, z): 
      x_coefs = poly.polyfit(x, z, 1)
      xfit = poly.Polynomial(x_coefs)    # instead of np.poly1d
      y_coefs = poly.polyfit(y, z, 3)
      yfit = poly.Polynomial(y_coefs)    # instead of np.poly1d
      return xfit, yfit 
  
  x = [3.08,  3.1 ,  3.12,  3.14,  3.16,  3.18,  3.2 ,  3.22,  3.24, 
       3.26,  3.28,  3.3 ,  3.32,  3.34,  3.36,  3.38,  3.4 ,  3.42, 
       3.44,  3.46,  3.48,  3.5 ,  3.52,  3.54,  3.56,  3.58,  3.6 , 
       3.62,  3.64,  3.66,  3.68]
  
  y = [0.000857,  0.001182,  0.001619,  0.002113,  0.002702,  0.003351,
      0.004062,  0.004754,  0.00546 ,  0.006183,  0.006816,  0.007362,
      0.007844,  0.008207,  0.008474,  0.008541,  0.008539,  0.008445,
      0.008251,  0.007974,  0.007608,  0.007193,  0.006752,  0.006269,
      0.005799,  0.005302,  0.004822,  0.004339,  0.00391 ,  0.003481,
      0.003095]
  
  z = np.linspace(0, 1, num=len(x)) # create as many z points as input data 
  
  xfit, yfit = fitData(x, y, z)
  
  ax1 = plot.subplot(1, 2, 1)
  plot.title("X position")
  plot.xlabel("Z (ft)")
  plot.ylabel("X (ft)")
  ax1.plot(z, xfit(z))
  
  ax2 = plot.subplot(1, 2, 2)
  plot.title("Y position")
  plot.xlabel("Z (ft)")
  plot.ylabel("Y (ft)")
  ax2.plot(z, yfit(z))
  
  plot.tight_layout()
  plot.show() 
  
  </code></pre>
  
  <p>And here are my resultant plots: </p>
  
  <p><a href="https://i.stack.imgur.com/fbOun.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/fbOun.png" alt="enter image description here"></a></p>
  
  <p>As you can see, the range on the X plot scale bar seems to run reverse, and the y plot scale is way off. Does anyone have any idea why this might be happening?</p>