Dotplot

What is a Dotplot?

Comic showing books counted as stacked dots over time.

Short Version: A Cleveland dotchart compares numeric values for named items. Each item has its own row, and the horizontal position of its dot shows its value.

Long Version: The term “dot plot” is used for several kinds of chart. In a stacked frequency dot plot, dots above the same value or bin count observations. The comic uses stacked dots to count books over time. The R examples below instead use Cleveland dotcharts: each car model has a separate row, and its dot shows miles per gallon on a numeric horizontal axis. The vertical positions identify the models; they do not measure frequencies. A strip plot is another arrangement, which places individual observations along a numeric axis, optionally separated into groups.

Create Cleveland dotcharts with dotchart(x, labels=), where x is a numeric vector and labels supplies a label for each point. The groups= option takes a factor specifying the groups. Use gcolor= for the group labels, color= for the points and item labels, and cex= for their size. See the R documentation for dotchart().

Example with mtcars:

Not grouped or coloured; original row order:

data(mtcars) #loading mtcars
dotchart(mtcars$mpg,labels=row.names(mtcars),cex=.7,
         main="Gas Milage for Car Models",
         xlab="Miles Per Gallon")
Cleveland dotchart of mpg for all mtcars models, with one labelled row per car and mpg on the horizontal axis.
Dotplots without grouping and colouring.

Dotplot by gears

Grouped, Sorted and Colored, sort by mpg, group and color by gears:

x <- mtcars[order(mtcars$mpg),] # sort by mpg
x$gear <- factor(x$gear) # it must be a factor
x$color_gear[x$gear==3] <- "red"
x$color_gear[x$gear==4] <- "blue"
x$color_gear[x$gear==5] <- "darkgreen"
dotchart(x$mpg,labels=row.names(x),cex=.7,groups= x$gear,
         main="Gas Milage for Car Models\ngrouped by gear",
         xlab="Miles Per Gallon", gcolor="black", color=x$color_gear)

Dotplot by cylinder

Grouped, Sorted and Colored, sort by mpg, group and color by cylinders:

x <- mtcars[order(mtcars$mpg),] # sort by mpg
x$cyl <- factor(x$cyl) # must be a factor
x$color_cyl[x$cyl==4] <- "red" # setting red for 4 cylinder
x$color_cyl[x$cyl==6] <- "blue" #setting blue for 6 cylinder
x$color_cyl[x$cyl==8] <- "darkgreen" # setting green for 8 cylinder
dotchart(x$mpg,labels=row.names(x),cex=.7,groups= x$cyl,
         main="Gas Milage for Car Models\ngrouped by cylinder",
         xlab="Miles Per Gallon", gcolor="black", color=x$color_cyl)
Two Cleveland dotcharts group and colour mtcars by gear count or cylinder count; mpg is sorted within each group.
Cleveland dotcharts grouped and coloured by gear (left) or cylinder count (right), with mpg sorted within each group.

Difference between Dotplot and Scatterplot:

  • The Cleveland dotchart below compares one numeric variable, miles per gallon, across named car models. Each row identifies a car model; the dot’s horizontal position gives its mpg value. It is not a stacked frequency plot.

  • A scatterplot shows two numeric variables for each observation. Here, each point represents one car, with horsepower on the horizontal axis and miles per gallon on the vertical axis. The pattern can reveal trends, clusters and unusual observations. The axes do not by themselves establish a causal relationship.

Comparison: the dotchart shows labelled car models against mpg; the scatterplot shows mpg against horsepower, with generally lower mpg at higher horsepower.
Difference between Dotplot and Scatterplot.

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