e offset rises smoothly 
    from 0 to 1cm at half-way along the main curve, then back to 0
    (the main curve is shown as a white line).
  </p>
<div class="chunk" id="render-variable-offset"><div class="rcode"><div class="source"><pre class="knitr r"><span class="hl std">x</span> <span class="hl kwb">&lt;-</span> <span class="hl num">1</span><span class="hl opt">:</span><span class="hl num">4</span><span class="hl opt">/</span><span class="hl num">5</span>
<span class="hl std">y</span> <span class="hl kwb