title: Find separating curve between two types of points
body: <p>In the following plot, it looks like there is a clear separating curve between the 'x' and 'o'. It seems to be a smooth non-linear curve.</p>
  
  <p>How can I find this curve, and possibly attempt to fit it with some formula?</p>
  
  <p><a href="https://i.stack.imgur.com/U67Bw.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/U67Bw.png" alt="enter image description here"></a></p>