title: draw the sum value above the stacked bar in ggplot2
body: <p>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：</p>
  
  <pre><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()
  </code></pre>