title: Calculate slopes between two points
body: <p>How can I calculate the slope between two data points?</p>
  
  <p>My values look like this:</p>
  
  <pre><code>2010 |  2011 |  2012 |  2013
  23.5 |  4    |  0    |  1.5
  </code></pre>
  
  <p>and so on. So, to each year belongs a value - AUC.</p>