for generating offset curves
    for X-splines, where the width of the X-spline
    is allowed to vary along its length.
    The implementation is provided by the 
    <code>grid.offsetXspline</code> function in the 
    'vwline' package for R.
  </p>
    <div>
      <h2>Table of Contents:</h2>
      <ul style="list-style: none">
        <li>
          <a href="#intro">1. Introduction</a>
        </li>
        <li>
          <a href="#offset-curves">2. Offset curves</a>
        </li>
        <li>
          