_vars<span class="op">=</span><span class="fu"><a href="https://rdrr.io/r/base/c.html" class="external-link">c</a></span><span class="op">(</span><span class="st">"x1"</span>, <span class="st">"x3"</span><span class="op">)</span>,</span>
<span class="r-in">                        conf_int<span class="op">=</span><span class="cn">FALSE</span><span class="op">)</span></span>
<span class="r-in"></span>
<span class="r-in"><span class="co"># plot the curves</span></span>
<span class="r-in"><span class="fu"><a hr