title: How to make monotonic (increasing) smooth spline with smooth.spline() function?
body: <p>I have data that are strictly increasing and would like to fit a smoothing spline that is monotonically increasing as well with the <code>smooth.spline()</code> function if possible, due to the ease of use of this function. </p>
  
  <p>For example, my data can be effectively reproduced with the example: </p>
  
  <pre><code>testx &lt;- 1:100
  testy &lt;- abs(rnorm(length(testx)))^3
  testy &lt;- cumsum(testy)
  plot(testx,testy)
  sspl &lt;- smooth.spline(testx,testy)
  lines(sspl,col="blue")
  </code></pre>
  
  <p>which is not necessarily increasing everywhere. Any suggestions?</p>