title: Change the color of circle in matplotlib
body: <p>I'm plotting a circle, but I want to change the color of the circle in this code. Can someone help me?</p>
  <pre><code>import numpy as np
  import matplotlib.pyplot as plt
  
  x = np.linspace(-1.0, 1.0, 100)
  y = np.linspace(-1.0, 1.0, 100)
  
  X, Y = np.meshgrid(x,y)
  
  F = X**2 + Y**2 - 1.0
  
  fig, ax = plt.subplots()
  
  ax.contour(X,Y,F,[0])
  
  ax.set_aspect(1)
  
  
  plt.xlim(-1.25,1.25)
  plt.ylim(-1.25,1.25)
  
  plt.grid(linestyle='--')
  
  plt.savefig(&quot;plot_circle_matplotlib_03.png&quot;, bbox_inches='tight')
  
  plt.show()
  </code></pre>
  <p>I try put other the lines for instance <code>facecolor='auto'</code> and try to change the line
  <code>plt.savefig(&quot;plot_circle_matplotlib_03.png&quot;, bbox_inches='tight')</code></p>
  <p>put
  <code>plt.savefig(&quot;plot_circle_matplotlib_03.png&quot;, bbox_inches='tight', 'back')</code>
  but actually I don't now what is the function of this line.</p>