gganimate: make points stay several frames before and after - r

Question

I have a data.frame containing timestamped events of different kinds, geolocated. I know how to plot an animation of each event as a point, hour by hour, with gganimate (*). It would be something like:
df = data.frame("id" = runif(500, 1e6, 1e7),
'lat' = runif(500, 45, 45.1),
'long'= runif(500, 45, 45.1),
'datetime'= seq.POSIXt(from=Sys.time(), to=Sys.time()+1e5, length.out=500),
'hour'=format(seq.POSIXt(from=Sys.time(), to=Sys.time()+1e5, length.out=500), "%H"),
'event'=paste0("type", rpois(500, 1)))
ggplot(data=df) +
aes(x=long, y=lat, color=factor(event)) +
geom_point() +
transition_states(hour, state_length = 1, transition_length = 0)
Now I would like to make points "stay" longer on screen, for instance if an event is at 5:00pm, i want it to be displayed on the animation from 2pm until 8pm (3 frames before and after his position, and if possible fade in and out). I don't know how to do that with gganimate, I tried to use transition_length but it's making the points "move" and that makes no sense for me!
Thanks,
(*) Edit: I thought of adding 6 duplicated rows for each row, and modifying the hour by -1 to +3, but it's a lot heavier and can't deal with fade in/out
library(magrittr)
df %<>% mutate(hour = hour + 2) %>% bind_rows(df)
df %<>% mutate(hour = hour + 1) %>% bind_rows(df)
df %<>% mutate(hour = hour - 4) %>% bind_rows(df)
df %<>% mutate(hour = hour %% 24 )

Answer 1

You can use transition_components and specify 3 hours as the enter / exit length for each point.
Data:
set.seed(123)
n <- 50 # 500 in question
df = data.frame(
id = runif(n, 1e6, 1e7),
lat = runif(n, 45, 45.1),
long = runif(n, 45, 45.1),
datetime = seq.POSIXt(from=Sys.time(), to=Sys.time()+1e5, length.out=n),
hour = format(seq.POSIXt(from=Sys.time(), to=Sys.time()+1e5, length.out=n), "%H"),
event = paste0("type", rpois(n, 1)))
Code:
df %>%
mutate(hour = as.numeric(as.character(hour))) %>%
ggplot() +
aes(x=long, y=lat, group = id, color=factor(event)) +
# I'm using geom_label to indicate the time associated with each
# point & highlight the transition times before / afterwards.
# replace with geom_point() as needed
geom_label(aes(label = as.character(hour))) +
# geom_point() +
transition_components(hour,
enter_length = 3,
exit_length = 3) +
enter_fade() +
exit_fade() +
ggtitle("Time: {round(frame_time)}")
This approach works with a datetime variable as well:
df %>%
ggplot() +
aes(x = long, y = lat, group = id, color = factor(event)) +
geom_label(aes(label = format(datetime, "%H:%M"))) +
transition_components(datetime,
enter_length = as_datetime(hm("3:0")),
exit_length = as_datetime(hm("3:0"))) +
enter_fade() +
exit_fade() +
ggtitle("Time: {format(frame_time, '%H:%M')}")

Answer 2

gganimate does not appear to be set up to handle leaving points on the plot. I think that you are going to have to go the manual route.
Here is a (slightly kludgy) approach to duplicate the rows including setting the times at which they should display and the offset (to be used for alpha to control fade):
df_withRange <-
df %>%
mutate(hour = parse_number(hour)) %>%
split(1:nrow(.)) %>%
lapply(function(x){
lapply(-3:3, function(this_time){
x %>%
mutate(frame_time = hour + this_time
, offset = this_time
, abs_offset = abs(this_time))
}) %>%
bind_rows()
}) %>%
bind_rows() %>%
mutate(
frame_time = ifelse(frame_time > 23, frame_time - 24, frame_time)
, frame_time = ifelse(frame_time < 0, frame_time + 24, frame_time)
)
Then, this code set up the plot:
ggplot(data=df_withRange
, aes(x=long
, y=lat
, color=factor(event)
, alpha = abs_offset
)) +
geom_point() +
transition_states(frame_time) +
labs(title = 'Hour: {closest_state}') +
scale_alpha_continuous(range = c(1,0.2))
The plot:
There is still a lot of clean up to do (e.g., the fade levels, etc.), but that should be a start at least