More tweaks from @behrman

visualize.Rmd

@@ -212,11 +212,6 @@ ggplot(shapes, aes(x, y)) +
 1.  What happens if you set an aesthetic to something other than a variable 
     name, like `displ < 5`?
 
-1.  Vignettes are long-form guides that document things about 
-    a package that affect many functions. ggplot2 has two vignettes.
-    How can you find them and what do they describe? (Hint: Google is
-    your friend.)
-
 ## Common problems
 
 As you start to run R code, you're likely to run into problems. Don't worry --- it happens to everyone. I have been writing R code for years, and every day I still write code that doesn't work! 
@@ -615,8 +610,7 @@ You can avoid this gridding by setting the position adjustment to "jitter".  `po
 
 ```{r}
 ggplot(data = mpg) + 
-  geom_point(mapping = aes(x = displ, y = hwy), position = "jitter") + 
-  ggtitle('Position = "jitter"')
+  geom_point(mapping = aes(x = displ, y = hwy), position = "jitter")
 ```
 
 Adding randomness seems like a strange way to improve your plot, but while it makes your graph less accurate at small scales, it makes your graph _more_ revealing at large scales. Because this is such a useful operation, ggplot2 comes with a shorthand for `geom_point(position = "jitter")`: `geom_jitter()`.
@@ -680,8 +674,7 @@ There are a number of other coordinate systems that are occasionally helpful.
         width = 1
       ) + 
       theme(aspect.ratio = 1) +
-      xlab(NULL) + 
-      ylab(NULL)
+      labs(x = NULL, y = NULL)
     
     bar + coord_flip()
     bar + coord_polar()
@@ -697,6 +690,8 @@ knitr::include_graphics("images/visualization-coordinate-systems.png")
 
 1.  Turn a stacked bar chart into a pie chart using `coord_polar()`.
 
+1.  What does `labs()` do? Read the documentation.
+
 1.  What's the difference between `coord_quickmap()` and `coord_map()`?
 
 1.  What does the plot below tell you about the relationship between city