 effects</code></pre>
<pre><code>## lm(formula = mpg ~ hp * wt, data = mtcars)</code></pre>
<pre><code>##        hp     wt
##  -0.03051 -4.132</code></pre>
<p>On average across the cases in the dataset, the effect of horsepower is slightly negative. On average, the effect of weight is also negative. Both decrease fuel economy. But what is the marginal effect of each variable across the range of values we actually observe in the data. To get a handle on this, we can use the <code>persp()</code> method provid