尝试使用 ggplot2 在同一图表上制作 2 个箱线图

Question

I am trying to complete an assignment using some fake data. One thing they would like us to do is make graphs.

The data is time for students n=150 to complete a task pre and post an intervention.

I have been able to make graphs like this

Boxplot1 <- ggplot(data, aes(Timepre))
Boxplot1 <- Boxplot1 + geom_boxplot()
Boxplot1
Boxplot2 <- ggplot(data, aes(Timepost))
Boxplot2 <- Boxplot2 + geom_boxplot()
Boxplot2

But I would like both to be on the same graph

How can I combine these please?

Answer 1

From your example I gather you have your data as two vectors with values. The most difficult part about this question is probably getting the data in the right shape.

Below, I'll make two vectors of length 150 with some random values.

library(ggplot2)

Timepre <- rpois(150, 30)
Timepost <- rpois(150, 20)

ggplot likes to work nicely with data that is in the so called 'long' format, meaning that every observation gets a row of its own in a data.frame . We'd need an extra variable, which we'll call x , to keep track of the groups.

df <- data.frame(
  x = rep(c("Pre", "Post"), c(length(Timepre), length(Timepost))),
  y = c(Timepre, Timepost)
)

# You might want to encode the groups as factors with levels in sensible orders
df$x <- factor(df$x, levels = c("Pre", "Post"))

After reshaping the data, plotting is not so difficult and you can probably find hundreds of posts on this site that can show you how to do it.

ggplot(df, aes(x, y)) +
  geom_boxplot()

^{Created on 2021-01-31 by the reprex package (v0.3.0)}