reorder factors for boxplot

It is very common that you want to reorder the boxplot according to the medians to see a better trend. I will show you how to do it using ggplot2 and the forcats packages which are developed by Hadely Wickham.

Read his new R for data science book: http://r4ds.had.co.nz/factors.html

head(iris)

a basic boxplot:

library(ggplot2)
ggplot(iris, aes(x = Species, y = Sepal.Width)) + geom_boxplot()

Now, reorder it according to the median levels.

ggplot(iris, aes(x = reorder(Species, Sepal.Width, FUN = median), y = Sepal.Width)) + geom_boxplot()

use the fct_reorder function from forcats. it has a similar syntax with reorder, but note that the argument fun is lower case.

library(forcats)
ggplot(iris, aes(x = fct_reorder(Species, Sepal.Width, fun = median), y = Sepal.Width)) + geom_boxplot()

You can change the order from high to low

ggplot(iris, aes(x = fct_reorder(Species, Sepal.Width, fun = median, .desc =TRUE), y = Sepal.Width)) + geom_boxplot()

some touch-ups

ggplot(iris, aes(x = fct_reorder(Species, Sepal.Width, fun = median, .desc =TRUE), y = Sepal.Width)) + 
        geom_boxplot(aes(fill = fct_reorder(Species, Sepal.Width, fun = median, .desc =TRUE))) + 
        geom_jitter(position=position_jitter(0.2)) +
        theme_bw(base_size = 14) +
        xlab("Species") +
        ylab("Sepal width") +
        scale_fill_discrete(guide = guide_legend(title = "Species"))

NA

If you want to fill the colors mannually with colors from RcolorBrewer:

library(RColorBrewer)
ggplot(iris, aes(x = fct_reorder(Species, Sepal.Width, fun = median, .desc =TRUE), y = Sepal.Width)) + 
        geom_boxplot(aes(fill = fct_reorder(Species, Sepal.Width, fun = median, .desc =TRUE))) + 
        scale_fill_manual(values = brewer.pal(3, "Dark2"), guide = guide_legend(title = "Species")) +
        geom_jitter(position=position_jitter(0.2)) +
        theme_bw(base_size = 14) +
        xlab("Species") +
        ylab("Sepal width") 

A different theme:

ggplot(iris, aes(x = fct_reorder(Species, Sepal.Width, fun = median, .desc =TRUE), y = Sepal.Width)) + 
        geom_boxplot(aes(fill = fct_reorder(Species, Sepal.Width, fun = median, .desc =TRUE))) + 
        scale_fill_manual(values = brewer.pal(3, "Dark2"), guide = guide_legend(title = "Species")) +
        geom_jitter(position=position_jitter(0.2)) +
        theme_classic(base_size = 14) +
        xlab("Species") +
        ylab("Sepal width")