Connect medians with lines in dodged boxplots within subgroups? - r

Question

In the code below, which uses an interaction of two variables to define a group, why ggplot2 correctly plots separate points for the medians, but complains that the group consists of only one observation when I try to plot lines connecting the medians within each group?
Please how can I do it?
dat <-
data.frame(method = c("A","A","A","A","A","A","A","A","A","A","B","B","B","B","B","B","B","B","B","B"),
var = c("X01","X01","X01","X01","X01","X02","X02","X02","X02","X02","X01","X01","X01","X01","X01","X02","X02","X02","X02","X02"),
value = c(-1.1,0.3,0.3,-0.6,-0.2,0.7,1.2,0.2,0.3,1.8,0.5,1.1,-0.8,0.8,0.6,1.0,0.5,0.6,1.5,1.3))
ggplot(dat, aes(var, value)) +
geom_boxplot(aes(fill = method),
position = position_dodge(width = .9),
outlier.color = NA) +
stat_summary(
fun = median,
geom = "point",
aes(group = interaction(method, var)),
position = position_dodge(width = .9)
) +
stat_summary(
fun = median,
geom = "line",
aes(group = interaction(method, var)),
position = position_dodge(width = .9)
)
geom_path: Each group consists of only one observation. Do you need to adjust
the group aesthetic?
Desired output:

Answer 1

Because ggplot2 thinks the groups are a combination of method and var. It doesn't know that after you've calculated the median, you'd like to drop the method part of the grouping. I don't think there is an elegant way to get the lines as you want.
Below is an inelegant put pragmatic solution: you first calculate the medians, then feed that layer data to a new line geom.
library(ggplot2)
dat <-
data.frame(method = c("A","A","A","A","A","A","A","A","A","A","B","B","B","B","B","B","B","B","B","B"),
var = c("X01","X01","X01","X01","X01","X02","X02","X02","X02","X02","X01","X01","X01","X01","X01","X02","X02","X02","X02","X02"),
value = c(-1.1,0.3,0.3,-0.6,-0.2,0.7,1.2,0.2,0.3,1.8,0.5,1.1,-0.8,0.8,0.6,1.0,0.5,0.6,1.5,1.3))
p <- ggplot(dat, aes(var, value)) +
geom_boxplot(aes(fill = method),
position = position_dodge(width = .9),
outlier.color = NA) +
stat_summary(
fun = median,
geom = "point",
aes(group = interaction(method, var)),
position = position_dodge(width = .9)
)
p + geom_line(
data = layer_data(p, 2),
aes(x = x, group = round(x), y = y)
)
Created on 2022-08-14 by the reprex package (v2.0.1)