Mapping in consistent place

Fixes #246

visualize.Rmd

@@ -236,8 +236,8 @@ One way to add additional variables is with aesthetics. Another way, particularl
 To facet your plot by a single variable, use `facet_wrap()`. The first argument of `facet_wrap()` should be a formula, which you create with `~` followed by a variable name (here "formula" is the name of a data structure in R, not a synonym for "equation"). The variable that you pass to `facet_wrap()` should be discrete. 
 
 ```{r}
-ggplot(data = mpg, mapping = aes(x = displ, y = hwy)) + 
-  geom_point() + 
+ggplot(data = mpg) + 
+  geom_point(mapping = aes(x = displ, y = hwy)) + 
   facet_wrap(~ class, nrow = 2)
 ```