title: Plot equation showing a circle
body: <p>The following formula is used to classify points from a 2-dimensional space:</p>
  
  <pre><code>f(x1,x2) = np.sign(x1^2+x2^2-.6)
  </code></pre>
  
  <p>All points are in space <code>X = [-1,1] x [-1,1]</code> with a uniform probability of picking each x.</p>
  
  <p>Now I would like to visualize the circle that equals:</p>
  
  <pre><code>0 = x1^2+x2^2-.6
  </code></pre>
  
  <p>The values of x1 should be on the x-axis and values of x2 on the y-axis.</p>
  
  <p>It must be possible but I have difficulty transforming the equation to a plot.</p>