at</code> argument is incredibly useful for getting a better grasp of the marginal effects of different covariates.</p>
<p>This becomes especially apparent when a model includes power-terms (or any other alternative functional form of a covariate). Consider, for example, the simple model of fuel economy as a function of weight, with weight included as both a first- and second-order term:</p>
<div class="sourceCode"><pre class="sourceCode r"><code class="sourceCode r">x &lt;-<span class="st"> </span><span cl