# Cars example

From statmethods.net.

    # ggplot2 examples
    library(ggplot2)
    #use color brewer as default discrete colors
    scale_colour_discrete <- function(...) scale_color_brewer(palette="Set1", ...)
    scale_fill_discrete <- function(...) scale_fill_brewer(palette="Set1", ...)

    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"))
    head(mtcars)

    ##                    mpg  cyl disp  hp drat    wt  qsec vs        am   gear
    ## Mazda RX4         21.0 6cyl  160 110 3.90 2.620 16.46  0    Manual 4gears
    ## Mazda RX4 Wag     21.0 6cyl  160 110 3.90 2.875 17.02  0    Manual 4gears
    ## Datsun 710        22.8 4cyl  108  93 3.85 2.320 18.61  1    Manual 4gears
    ## Hornet 4 Drive    21.4 6cyl  258 110 3.08 3.215 19.44  1 Automatic 3gears
    ## Hornet Sportabout 18.7 8cyl  360 175 3.15 3.440 17.02  0 Automatic 3gears
    ## Valiant           18.1 6cyl  225 105 2.76 3.460 20.22  1 Automatic 3gears
    ##                   carb
    ## Mazda RX4            4
    ## Mazda RX4 Wag        4
    ## Datsun 710           1
    ## Hornet 4 Drive       1
    ## Hornet Sportabout    2
    ## Valiant              1

## 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, transmission 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")

## Regressions of mpg on weight

Separate 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")

## 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")

Author: Jim Hester Created: 2013 Mar 20 10:57:07 AM Last Modified: 2014 Apr 17 04:53:11 PM