library(tidyverse)
library(gganimate)

tbl <- tibble(x = seq(-8, 8, by = .01),
             A_1 = 4*x,
             B_1 = x^2,
             A_2 = 3*x,
             B_2 = x^2,
             A_3 = 2*x,
             B_3 = x^2,
             A_4 = x,
             B_4 = x^2,
             A_5 = 0*x,
             B_5 = x^2) %>%
 gather(group, density, A_1:B_5) %>%
 separate(group, c("group", "type"), sep = "_") %>%
 mutate(type = as.numeric(type)) %>%
 mutate(Title = case_when(
   type == 1 ~ "A = 0, B = 4",
   type == 2 ~ "A = 0, B = 3",
   type == 3 ~ "A = 0, B = 2",
   type == 4 ~ "A = 0, B = 1",
   TRUE ~ "A = B = 0"
 ))


  ggplot(tbl) + geom_line(mapping = aes(x = x, y = density, colour = group)) +
 transition_states(Title, transition_length = .5, state_length = 2, wrap = TRUE) +
 labs(title = '{closest_state}') + ylab("f(x)")