Sorting Within Lattice Graphics in R

November 29, 2012
By

(This article was first published on Mollie's Research Blog, and kindly contributed to R-bloggers)

Default

By default, lattice sorts the observations by the axis values, starting at the bottom left.

For example,

library(lattice)
colors = c("#1B9E77", "#D95F02", "#7570B3")
dotplot(rownames(mtcars) ~ mpg,
data = mtcars,
col = colors[1],
pch = 1)

produces:

Default lattice dotplot

(Note: The rownames(cars) bit is just because of how this data set is stored. For your data, you might just type the variable name (model, for example) instead.)

Graphing one variable, sorting by another

If we want to show the same data, but we want to sort by another variable (or the same variable, for that matter), we can just add reorder(yvar, sortvar). For example, to sort by the number of cylinders, we could:

dotplot(reorder(rownames(mtcars), cyl) ~ mpg,
data = mtcars,
col = colors[1],
pch = 1)
Sorted by number of cylinders

Graphing two variables

To better show how this works, let’s graph cyl alongside mpg, so we can see how it is sorting:

dotplot(reorder(rownames(mtcars), cyl) ~ mpg + cyl,
data = mtcars,
col = colors,
pch = c(1, 0))
Graph of mpg and cyl, sorted by cyl

Reverse order

We can also sort in reverse order, by adding a “-” before the variable name:

dotplot(reorder(rownames(mtcars), -cyl) ~ mpg + cyl,
data = mtcars,
col = colors,
pch = c(1, 0))
Graph of mpg and cyl, sorted by cyl, reversed

Adding a legend

We can also add a legend:

dotplot(reorder(rownames(mtcars), -cyl) ~ mpg + cyl,
data = mtcars,
xlab = "",
col = colors,
pch = c(1, 0),
key = list(points = list(col = colors[1:2], pch = c(1, 0)),
text = list(c("Miles per gallon", "Number of cylinders")),
space = "bottom"))
With legend

Other lattice types

The same technique will work with other lattice graphs, such as barchart, bwplot, and stripplot.

Full code available as a gist.

To leave a comment for the author, please follow the link and comment on their blog: Mollie's Research Blog.

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