Slanted x-axis labels for boxplots

Question

I need the x-axis labels to be slanted at 45 degrees. Also, how do I reduce the number of boxplots shown in each visual without changing the source data?

I know the code I need to add is srt = 45 but where? Also, how do I change code below so that each visual only shows 3 boxplots?

boxplot(Transport$mph ~ Transport$CarType, main = "Mph by Car Type",
    xlab = "Car Type", ylab= "Mph", col= "grey")

Currently, the x-axis labels are horizontal so not all the labels are showing. I want them to be slanted at 45 degrees so that all the labels can be seen. Also, I want to know how to specify smaller quantity of boxplots in each visual as there are too many boxplots in one visual currently. I am happy to have many visuals showing only 3 boxplots each.

Answer 1

This example uses built in data set mtcars . The key is to not plot the x-axis labels, xaxt = "n" and then plot the labels with text .

labs <- seq_along(unique(mtcars$cyl))

boxplot(mpg ~ cyl, data = mtcars, xaxt = "n",
    main = "Mph by Car Type",
    xlab = "Car Type", ylab= "Mph", col= "grey")
text(seq_along(unique(mtcars$cyl)), par("usr")[3], 
    labels = labs, srt = 45, adj = c(1.1, 1.1), xpd = TRUE)

Answer 2

To customize axes in base plotting, you need to reconstruct them piece-by-piece:

data('mpg', package = 'ggplot2')

x_labs <- levels(factor(mpg$class))
boxplot(hwy ~ class, mpg, main = "Highway MPG by car type", 
        xlab = NULL, ylab = "Highway MPG", col = "grey", xaxt = 'n')    # don't plot axis
axis(1, labels = FALSE)    # add tick marks
text(x = seq_along(x_labs), y = 9, labels = x_labs, 
     srt = 45,    # rotate
     adj = 1,    # justify
     xpd = TRUE)    # plot in margin
mtext("Car Type", side = 1, padj = 6)    # add axis label

This is somewhat easier in ggplot, since it handles a lot of the alignment, keeping track of labels, etc. for you:

library(ggplot2)

ggplot(mpg, aes(class, hwy)) + 
    geom_boxplot(fill = 'grey') + 
    labs(title = "Highway MPG by car type", x = "Car type", y = "Highway MPG") +
    theme(axis.text.x = element_text(angle = 45, hjust = 1))