ar1, ...)</code>
</li>
</ul>
</li>
<li>
<p><code>alpha</code>: add an alpha transparency to the selected variable</p>
<ul>
<li>unless the variable is called in <code>x</code>, it will be added to the formula in the first position</li>
</ul>
</li>
<li>
<p><code>fill</code>: select a variable to be filled</p>
<ul>
<li>unless the variable is called in <code>x</code>, it will be added to the formula in the first position after the optional <code>alpha</code> variable.</li>
</ul>
</li>
<li>
<p><code>conds</code>