library(data.table)
library(gganimate)
library(ggplot2)

#toy example data
dt = data.table(time=1:10, x=round(runif(10, 50, 100), 0))

#number of frames per bar
n_frames_per_bar = 20
#create sequence per time (to imitate the bar growing from ground)
split_dt = split(dt, dt$time)
split_dt_fill = lapply(split_dt, function(dti) {
  #create sequence of length n per bar for animation
  data.table(time=dti$time, x=seq(1, dti$x, length.out = n_frames_per_bar))
})
dt_fill = rbindlist(split_dt_fill)

#id each row as its own frame
dt_fill[,frame := 1:.N]

#once a bar is grown it needs to stay there
#fill in historical bars at full height in future time periods
split_dt_fill2 = split(dt_fill, dt_fill$time)
split_dt_fill2_backfill = lapply(split_dt_fill2, function(dti){
  #get time i
  time_i = unique(dti$time)
  #make bar for time i a different color
  dti[,fill := "time i"]
  #backfill bar heights if there is history to fill in
  if (time_i > 1) {
    #get max bar height for all historical bars
    backfill_dt = dt_fill[time < time_i,][order(-frame), .SD[1], by=time][,.(time,x)]
    #create a row of max height for each frame in time i
    split_backfill_dt = split(backfill_dt, backfill_dt$time)
    split_backfill_dt = lapply(split_backfill_dt, function(dtj){
      data.table(time=dtj$time, x=dtj$x, frame=dti$frame)
    })
    backfill_dt = rbindlist(split_backfill_dt)
    #label fill group for coloring bars
    backfill_dt[,fill := "historical"]

    out = rbind(dti, backfill_dt)
  } else {
    out = dti
  }

  out
})
plot_dt = rbindlist(split_dt_fill2_backfill)

#plot (using gganimate's frame arg in aes)
p = ggplot(plot_dt, aes(x=time, y=x, frame=frame, fill=fill)) +
  geom_bar(stat="identity", position = "identity") +
  scale_fill_manual(values=c("steelblue", "steelblue1")) +
  guides(fill=FALSE) +
  labs(x="Time", y="Value", title="")
#create gif (using interval to specify the time per frame)
gganimate(p, title_frame = FALSE, interval = .001)