QuickR - Graphics with ggplot2

I have been asked about problems with the code from the Graphics with ggplot2 from Quick-R

library(ggplot2)

Note the use of the data() function. This reads the mtcars dataset into R cleanly before trying to re-label of the factors.

Sometimes there are problems running and rerunning ones code on the mtcars dataset.

data(mtcars)
# create factors with value labels
mtcars$gear <- factor(mtcars$gear,levels=c(3,4,5),
   labels=c("3gears","4gears","5gears"))
mtcars$am <- factor(mtcars$am,levels=c(0,1),
   labels=c("Automatic","Manual"))
mtcars$cyl <- factor(mtcars$cyl,levels=c(4,6,8),
   labels=c("4cyl","6cyl","8cyl"))
mtcars

# Kernel density plots for mpg
# grouped by number of gears (indicated by color)
qplot(mpg, data=mtcars, geom="density", fill=gear, alpha=I(.5),
   main="Distribution of Gas Milage", xlab="Miles Per Gallon",
   ylab="Density")

# Scatterplot of mpg vs. hp for each combination of gears and cylinders
# in each facet, transmittion type is represented by shape and color
qplot(hp, mpg, data=mtcars, shape=am, color=am,
   facets=gear~cyl, size=I(3),
   xlab="Horsepower", ylab="Miles per Gallon")

# Separate regressions of mpg on weight for each number of cylinders
qplot(wt, mpg, data=mtcars, geom=c("point", "smooth"),
   method="lm", formula=y~x, color=cyl,
   main="Regression of MPG on Weight",
   xlab="Weight", ylab="Miles per Gallon")

Ignoring unknown parameters: method, formula

# Boxplots of mpg by number of gears
# observations (points) are overlayed and jittered
qplot(gear, mpg, data=mtcars, geom=c("boxplot", "jitter"),
   fill=gear, main="Mileage by Gear Number",
   xlab="", ylab="Miles per Gallon") 

library(ggplot2)
p <- qplot(hp, mpg, data=mtcars, shape=am, color=am,
   facets=gear~cyl, main="Scatterplots of MPG vs. Horsepower",
   xlab="Horsepower", ylab="Miles per Gallon")
# White background and black grid lines
p + theme_bw()

# Large brown bold italics labels
# and legend placed at top of plot
p + theme(axis.title=element_text(face="bold.italic",
   size="12", color="brown"), legend.position="top")