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! 
@@ -516,7 +511,7 @@ Stats are the most subtle part of plotting because you can't see them directly.
     is generated by counting rows.)
 
 1.  You might want to override the default mapping from transformed variables
-    to aesthetics.  For example, you might want to display a bar chart of
+    to aesthetics. For example, you might want to display a bar chart of
     proportion, rather than count:
     
     ```{r}
@@ -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