您需要重新排列总和函数，否则它默认使用 mean 函数。然后，我在 amount 前面放置了一个 - 以使订单逆转。

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

I have problem in sorting x axis by y value in ggplot2: here is the code below

#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,amount),y=amount,label=amount,fill=year)+
theme()

Here is the result:

How do I sort my x axis by a c b d, which amount sorted by decreasing from 450, 290, 150, 90. What should I do?

You need to give reorder the sum function, otherwise it defaults to using the mean function. Then, I put a - in front of amount to get the order reversed.

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