 <p>You cannot change line size for <code>sjp.int</code> currently, so you have to modify the returned plot object(s), which are in the return value <code>plot.list</code>. Then you can overwrite <code>geom_line()</code>.</p> <pre><code>dummy &lt;- sjp.int(test, type = "eff") dummy$plot.list[[1]] + geom_line(size = 3) </code></pre> <p><a href="https://i.stack.imgur.com/6AA66.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/6AA66.png" alt="enter image description here"></a></p> <p>I've added a <code>geom.size</code> argument to <code>sjp.int</code>, see <a href="https://github.com/sjPlot/devel" rel="nofollow noreferrer">GitHub</a>.</p>