ggplot line plot with one group`s lines on top - r

Question

I am making a line plot of several groups and want to make a visualization where one of the groups lines are highlighted
ggplot(df) + geom_line(aes(x=timepoint ,y=var, group = participant_id, color=color)) +
scale_color_identity(labels = c(red = "g1",gray90 = "Other"),guide = "legend")
However, the group lines are partially obscured by the other groups lines
How can I make these lines always on top of other groups lines?

Answer 1

The simplest way to do this is to plot the gray and red groups on different layers.
First, let's try to replicate your problem with a dummy data set:
set.seed(1)
df <- data.frame(
participant_id = rep(1:50, each = 25),
timepoint = factor(rep(0:24, 50)),
var = c(replicate(50, runif(1, 50, 200) + runif(25, 0.3, 1.5) *
sin(0:24/(0.6*pi))^2/seq(0.002, 0.005, length = 25))),
color = rep(sample(c("red", "gray90"), 50, TRUE, prob = c(1, 9)), each = 100)
)
Now we apply your plotting code:
library(ggplot2)
ggplot(df) +
geom_line(aes(x=timepoint ,y=var, group = participant_id, color = color)) +
scale_color_identity(labels = c(red = "g1", gray90 = "Other"),
guide = "legend") +
theme_classic()
This looks broadly similar to your plot. If instead we plot in different layers, we get:
ggplot(df, aes(timepoint, var, group = participant_id)) +
geom_line(data = df[df$color == "gray90",], aes(color = "Other")) +
geom_line(data = df[df$color == "red",], aes(color = "gl")) +
scale_color_manual(values = c("red", "gray90")) +
theme_classic()
Created on 2022-06-20 by the reprex package (v2.0.1)

Answer 2

You can use factor releveling to bring the line (-s) of interest to front.
First, let's plot the data as is, with the red line partly hidden by others.
library(ggplot2)
library(dplyr)
set.seed(13)
df <-
data.frame(timepoint = rep(c(1:100), 20),
participant_id = paste0("p_", sort(rep(c(1:20), 100))),
var = abs(rnorm(2000, 200, 50) - 200),
color = c(rep("red", 100), rep("gray90", 1900)))
ggplot(df) +
geom_line(aes(x = timepoint ,
y = var,
group = participant_id, color = color)) +
scale_color_identity(labels = c(red = "g1", gray90 = "Other"),
guide = "legend")
Now let's bring p_1 to front by making it the last factor level.
df %>%
mutate(participant_id = factor(participant_id)) %>%
mutate(participant_id = relevel(participant_id, ref = "p_1")) %>%
mutate(participant_id = factor(participant_id, levels = rev(levels(participant_id)))) %>%
ggplot() +
geom_line(aes(x=timepoint,
y=var,
group = participant_id,
color = color)) +
scale_color_identity(labels = c(red = "g1", gray90 = "Other"),
guide = "legend")