title: Calculate the Area under a Curve
body: <p>I would like to calculate the area under a curve to do integration without defining a function such as in <code>integrate()</code>.</p>
  
  <p>My data looks as this:</p>
  
  <pre><code>Date          Strike     Volatility
  2003-01-01    20         0.2
  2003-01-01    30         0.3
  2003-01-01    40         0.4
  etc.
  </code></pre>
  
  <p>I plotted <code>plot(strike, volatility)</code> to look at the volatility smile. Is there a way to integrate this plotted "curve"?</p>