Review of Basic Data Analytic Methods Using R - Data Science and Big Data Analytics - page 130

Dotchart and Barplot
Dotchart and barplot from the previous section can visualize multiple variables.
Both of them use color as an additional dimension for visualizing the data.
For the same mtcars dataset, Figure 3.14 shows a dotchart that groups vehicle
cylinders at the y-axis and uses colors to distinguish different cylinders. The
vehicles are sorted according to their MPG values. The code to generate Figure 3.14
is shown next.
# sort by mpg
cars <- mtcars[order(mtcars$mpg),]
# grouping variable must be a factor
cars$cyl <- factor(cars$cyl)
cars$color[cars$cyl==4] <- "red"
cars$color[cars$cyl==6] <- "blue"
cars$color[cars$cyl==8] <- "darkgreen"
dotchart(cars$mpg, labels=row.names(cars), cex=.7, groups=
cars$cyl,
main="Miles Per Gallon (MPG) of Car Models\nGrouped by
Cylinder",
xlab="Miles Per Gallon", color=cars$color, gcolor="black")