lative to
    five control points.  The top X-spline has a shape parameter of 1 
    (except at the end points) so it approximates the control points.
    The bottom X-spline has a shape parameter of -1
    (except at the end points) so it interpolates the control points.
    The middle X-spline has a shape parameter of 0, so it produces
    straight lines and sharp corners.
  </p>


    <p class="img">
      <img src="figure/x-spline-1.svg" width="30%">
    </p>
    <p>
    The R graphics system provides f