如何在R的多列中绘制geom_area()

df <- data.frame(
  time = c(1, 2, 3, 4, 5),
  var1 = c(10, 15, 8, 12, 6),
  var2 = c(5, 3, 7, 9, 11),
  var3 = c(2, 6, 4, 8, 3)
)

library(tidyverse)

df <- df %>%
  gather(variable, value, -time)

   time variable value
1     1     var1    10
2     2     var1    15
3     3     var1     8
4     4     var1    12
5     5     var1     6
6     1     var2     5
7     2     var2     3
8     3     var2     7
9     4     var2     9
10    5     var2    11
11    1     var3     2
12    2     var3     6
13    3     var3     4
14    4     var3     8
15    5     var3     3

library(ggplot2)

ggplot(df, aes(x = time, y = value, fill = variable)) +
  geom_area()