have paths fade away in gganimate - r

Question

I am trying to plot multiple paths in a gganimate plot. I want the lines to fade out over the last N frames (e.g. N=5 in this example).
The data look like this:
set.seed(27)
df <- data.frame(Frame = rep(1:10, 3),
id = factor(rep(1:3, each = 10)),
x = runif(30),
y = runif(30))
head(df)
Frame id x y
1 1 1 0.97175023 0.14257923
2 2 1 0.08375751 0.47864658
3 3 1 0.87386992 0.05182206
4 4 1 0.32923136 0.25514379
5 5 1 0.22227551 0.14262912
6 6 1 0.40164822 0.48288482
I tried to make the plot using shadow_mark, but this doesn't appear to have the lines fade out over time.
df %>%
ggplot(aes(x = x, y = y, group = id, color = id)) +
geom_path() +
geom_point()+
scale_color_manual(values=c("red","blue","green")) +
transition_reveal(along = Frame) +
shadow_mark(size = 0.75) +
theme_void()
This just produces the below:
Is there a way to make these lines fade. Ideally, I'm just plotting a rolling path of N frames.

Answer 1

Is this something like what you're looking for? Adapted from the post mentioned in the comments. You don't need to use transition_reveal() if you use geom_segment().
library(gganimate)
library(dplyr)
library(tidyr)
set.seed(27)
n <- 10
df <- data.frame(Frame = rep(1:n, 3),
id = factor(rep(1:3, each = n)),
x = runif(3*n),
y = runif(3*n))
newdf <- df %>%
uncount(n, .id = "newframe") %>%
filter(Frame <= newframe) %>%
arrange(newframe, Frame) %>%
group_by(newframe, id) %>%
mutate(x_lag = lag(x),
y_lag = lag(y),
tail = last(Frame) - Frame,
# Make the points solid for 1 frame then alpha 0
point_alpha = if_else(tail == 0, 1, 0),
# Make the lines fade out over 3 frames
segment_alpha = pmax(0, (3-tail)/3)) %>%
ungroup()
ggplot(newdf,
aes(x = y, y = x, xend = y_lag, yend = x_lag, group = Frame, color = id)) +
geom_segment(aes(alpha = segment_alpha)) +
geom_point(aes(alpha = point_alpha)) +
scale_alpha(range = c(0,1)) +
guides(alpha = F) +
transition_manual(newframe) +
theme_void() +
scale_color_manual(values = c("red","blue","green"))