iv class="ref-description section level2">
    <p>Add vertical lines to overlay the surface of an x3p object. Lines are added to a mask. In case no mask exists, one is created.</p>
    </div>

    <div class="section level2">
    <h2 id="ref-usage">Usage<a class="anchor" aria-label="anchor" href="#ref-usage"></a></h2>
    <div class="sourceCode"><pre class="sourceCode r"><code><span><span class="fu">x3p_add_vline</span><span class="op">(</span><span class="va">x3p</span>, <span class="va">xintercept</span>,