In a dot plot, the width of a dot corresponds to the bin width (or maximum width, depending on the binning algorithm), and dots are stacked, with each dot representing one observation.
geom_dotplot(mapping = NULL, data = NULL, position = "identity", ..., binwidth = NULL, binaxis = "x", method = "dotdensity", binpositions = "bygroup", stackdir = "up", stackratio = 1, dotsize = 1, stackgroups = FALSE, origin = NULL, right = TRUE, width = 0.9, drop = FALSE, na.rm = FALSE, show.legend = NA, inherit.aes = TRUE)
mapping  Set of aesthetic mappings created by 

data  The data to be displayed in this layer. There are three options: If A A 
position  Position adjustment, either as a string, or the result of a call to a position adjustment function. 
...  Other arguments passed on to 
binwidth  When 
binaxis  The axis to bin along, "x" (default) or "y" 
method  "dotdensity" (default) for dotdensity binning, or "histodot" for fixed bin widths (like stat_bin) 
binpositions  When 
stackdir  which direction to stack the dots. "up" (default), "down", "center", "centerwhole" (centered, but with dots aligned) 
stackratio  how close to stack the dots. Default is 1, where dots just just touch. Use smaller values for closer, overlapping dots. 
dotsize  The diameter of the dots relative to 
stackgroups  should dots be stacked across groups? This has the effect
that 
origin  When 
right  When 
width  When 
drop  If TRUE, remove all bins with zero counts 
na.rm  If 
show.legend  logical. Should this layer be included in the legends?

inherit.aes  If 
There are two basic approaches: dotdensity and histodot.
With dotdensity binning, the bin positions are determined by the data and
binwidth
, which is the maximum width of each bin. See Wilkinson
(1999) for details on the dotdensity binning algorithm. With histodot
binning, the bins have fixed positions and fixed widths, much like a
histogram.
When binning along the x axis and stacking along the y axis, the numbers on y axis are not meaningful, due to technical limitations of ggplot2. You can hide the y axis, as in one of the examples, or manually scale it to match the number of dots.
geom_dotplot
understands the following aesthetics (required aesthetics are in bold):
x
y
alpha
colour
fill
group
Learn more about setting these aesthetics in vignette("ggplot2specs")
center of each bin, if binaxis is "x"
center of each bin, if binaxis is "x"
max width of each bin if method is "dotdensity"; width of each bin if method is "histodot"
number of points in bin
count, scaled to maximum of 1
density of points in bin, scaled to integrate to 1, if method is "histodot"
density, scaled to maximum of 1, if method is "histodot"
Wilkinson, L. (1999) Dot plots. The American Statistician, 53(3), 276281.
#># Use fixedwidth bins ggplot(mtcars, aes(x = mpg)) + geom_dotplot(method="histodot", binwidth = 1.5)# Some other stacking methods ggplot(mtcars, aes(x = mpg)) + geom_dotplot(binwidth = 1.5, stackdir = "center")# y axis isn't really meaningful, so hide it ggplot(mtcars, aes(x = mpg)) + geom_dotplot(binwidth = 1.5) + scale_y_continuous(NULL, breaks = NULL)# Overlap dots vertically ggplot(mtcars, aes(x = mpg)) + geom_dotplot(binwidth = 1.5, stackratio = .7)# Examples with stacking along y axis instead of x ggplot(mtcars, aes(x = 1, y = mpg)) + geom_dotplot(binaxis = "y", stackdir = "center")#>#>ggplot(mtcars, aes(x = factor(cyl), y = mpg)) + geom_dotplot(binaxis = "y", stackdir = "centerwhole")#>ggplot(mtcars, aes(x = factor(vs), fill = factor(cyl), y = mpg)) + geom_dotplot(binaxis = "y", stackdir = "center", position = "dodge")#># binpositions="all" ensures that the bins are aligned between groups ggplot(mtcars, aes(x = factor(am), y = mpg)) + geom_dotplot(binaxis = "y", stackdir = "center", binpositions="all")#># Stacking multiple groups, with different fill ggplot(mtcars, aes(x = mpg, fill = factor(cyl))) + geom_dotplot(stackgroups = TRUE, binwidth = 1, binpositions = "all")