{r}(t) + d(t)\mathbf{n}(t)$$
  </p>
    <p>
    The function \(d(t)\) can in theory be any function that takes a 
    single numeric argument in the range 0 to 1.
    In the 'vwline' package, the offset can be specified
    using the <code>widthSpline</code> function.
    This describes the offset as an X-spline, where the x-values for
    the control points are distances along the main curve and the 
    y-values for the control points are amounts of offset.
    The following code gives an example where th