draw the sum value above the stacked bar in ggplot2

Question

How do I draw the sum value of each class (in my case: a=450, b=150, c=290, d=90) above the stacked bar in ggplot2？ Here is my code：

#Data
hp=read.csv(textConnection(
"class,year,amount
a,99,100
a,100,200
a,101,150
b,100,50
b,101,100
c,102,70
c,102,80
c,103,90
c,104,50
d,102,90"))
hp$year=as.factor(hp$year)

#Plotting
p=ggplot(data=hp)  
p+geom_bar(binwidth=0.5,stat="identity")+  
aes(x=reorder(class,-value,sum),y=value,label=value,fill=year)+
theme()

Answer 1

You can do this by creating a dataset of per-class totals (this can be done multiple ways but I prefer dplyr):

library(dplyr)
totals <- hp %>%
    group_by(class) %>%
    summarize(total = sum(value))

Then adding a geom_text layer to your plot, using totals as the dataset:

p + geom_bar(binwidth = 0.5, stat="identity") +  
    aes(x = reorder(class, -value, sum), y = value, label = value, fill = year) +
    theme() +
    geom_text(aes(class, total, label = total, fill = NULL), data = totals)

You can make the text higher or lower than the top of the bars using the vjust argument, or just by adding some value to total:

p + geom_bar(binwidth = 0.5, stat = "identity") +  
    aes(x = reorder(class, -value, sum), y = value, label = value, fill = year) +
    theme() +
    geom_text(aes(class, total + 20, label = total, fill = NULL), data = totals)