To be used with transition_reveal() when you have your own column for reveal time

split_track.R

library(tidyverse)
library(gganimate)
library(rlang)

split_track <- function(df, grouping_var, tracked_along, ..., tracked_groups = ".all") {
  
  region <- df %>% 
    filter(...) %>% 
    group_by({{ grouping_var }}) %>% 
    mutate(row_id = 1:n()) %>% 
    ungroup() %>% 
    select(row_id, {{ grouping_var }}, {{ tracked_along }})
  region_wide <- region %>% 
    pivot_wider(names_from = {{ grouping_var }}, values_from = {{ tracked_along }}) %>% 
    select(-row_id)
  
  if (tracked_groups == ".all") {
    tracked_groups <- names(region_wide)
    untracked <- NULL
  } else {
    untracked <- region_wide %>% 
      select(-{{ tracked_groups }})
  }
  
  grouping_var_enquo <- enquo(grouping_var)
  tracked_along_enquo <- enquo(tracked_along)
  
  region_split <<- bind_rows(
    untracked,
    map_dfr(tracked_groups, ~select(region_wide, .x))
  ) %>% 
    fill(everything(), .direction = "down") %>%
    mutate(across(
      everything(),
      ~ifelse(is.na(.x), min(pull(region, {{ tracked_along }}), na.rm = TRUE) - 1, .x))
    ) %>% 
    pivot_longer(
      everything(),
      names_to = quo_text(grouping_var_enquo),
      values_to = quo_text(tracked_along_enquo)
    )

  df %>%
    anti_join(filter(df, ...)) %>% 
    bind_rows(
      region_split %>%
        inner_join(df)
    ) %>% 
    group_by({{ grouping_var }}) %>% 
    mutate(!!tracked_along_enquo := row_number()) %>% 
    ungroup()

}


###################
## Generate Plot ##
###################

set.seed(123)

plot_df <- tibble(
  a = cumsum(runif(100, -1, 1)),
  b = cumsum(runif(100, -1.5, 1.5)), 
  c = cumsum(runif(100, -2, 2)),
  d = cumsum(runif(100, -2.5, 2.5))
) %>% pivot_longer(a:d) %>%
  group_by(name) %>%
  mutate(reveal_time = row_number(), x = row_number()) %>%
  ungroup()

anim <- plot_df %>%
  split_track(grouping_var = name, tracked_along = reveal_time, x > 74) %>%
  ggplot(aes(x, value, color = name)) +
  geom_line(size = 1) +
  transition_reveal(reveal_time) +
  theme_classic()

animate(anim, width = 5, height = 3, units = "in", res = 150)