title: When to use approxfun vs. approx
body: <p>The documentation for <code>approxfun</code> states that it is "often more useful than <code>approx</code>". I'm struggling to get my head around <code>approxfun</code>. When would <code>approxfun</code> be more useful than <code>approx</code> (and when would <code>approx</code> be more useful)?</p>