title: Compute the 'elbow' for a curve automatically and mathematically
body: <p>One example for curve is shown as below. The elbow point might be x=3 or 4.
  How to compute the elbow for a curve automatically and mathematically?</p>
  
  <p><img src="https://i.stack.imgur.com/3ETM1.jpg" alt="alt text"></p>