title: Plotting a Circular Graph on Python
body: <p>I have recently started using python scripting, but have come across a problem when trying to plot a simple graph. It is supposed to produce a Mohr's Circle, but I am definitely missing something. 
  This is the script I have at the moment: Any help would be greatly appreciated!</p>
  
  <pre><code>import math
  import numpy as np
  import matplotlib.pyplot as plt
  %pylab inline
  
  # max = maximum principal stress
  # min = minimum principal stress
  # x_norm = normal stress in x-direction
  # y_norm = normal stress in y-direction
  # t = shear stress
  
  x_norm = -60
  y_norm = 50
  t = 50
  
  R = math.sqrt(((x_norm - y_norm)/2)**2 + t**2)
  print R
  
  max = (x_norm + y_norm)/2 + R
  min = (x_norm + y_norm)/2 - R
  print max
  print min
  
  theta = 0.5 * math.atan((2*t)/(x_norm - y_norm))
  print theta
  
  aver = (x_norm + y_norm)/2
  print aver
  
  x = (min, max, x_norm, y_norm)
  y = (math.sqrt(((x_norm - y_norm)/2)**2 + t**2))
  plt.subplot(1,2,1)
  plt.plot(x,y)
  plt.show()
  </code></pre>
  
  <hr>
  
  <p>I have tried changing it, but get an error on the last part.</p>
  
  <pre><code>import math
  import numpy as np
  import matplotlib.pyplot as plt
  %pylab inline
  
  # max = maximum principal stress
  # min = minimum principal stress
  # x_norm = normal stress in x-direction
  # y_norm = normal stress in y-direction
  # t = shear stress
  
  x_norm = -60
  y_norm = 50
  t = 50
  
  R = math.sqrt(((x_norm - y_norm)/2)**2 + t**2)
  print R
  
  max = (x_norm + y_norm)/2 + R
  min = (x_norm + y_norm)/2 - R
  print max
  print min
  
  theta = 0.5 * math.atan((2*t)/(x_norm - y_norm))
  print theta
  
  aver = (x_norm + y_norm)/2
  print aver
  
  def circle(x_norm, y_norm, t):
      x = (min, max, x_norm, y_norm)
      y = (math.sqrt(((x_norm - y_norm)/2)**2 + t**2))
      plt.subplot(1,2,1)
      plt.plot(x,y)
      plt.show()
  
  circle(rad,x,y,np.linspace(-100,100,500))
  print("Done")
  
  
  
  NameError                                 Traceback (most recent call last)
  &lt;ipython-input-5-0b893be1e1e8&gt; in &lt;module&gt;()
       35     plt.show()
       36 
  ---&gt; 37 circle(rad,x,y,np.linspace(-100,100,500))
       38 print("Done")
  
  NameError: name 'x' is not defined
  </code></pre>