通过R中的两个变量绘制分组数据的媒体

frame that looks like this

看起来像这样的框架

df<- data.frame(samples=1:60, location=rep(letters[1:3], 20), 
                variable=c(rep("A", 20), rep("B", 20), rep("C", 20)), value=rnorm(60))

df
  samples location variable       value
1       1        a        A  0.32513726
2       2        b        A  2.05029602
3       3        c        A -0.10470011
4       4        a        A  0.17142511
5       5        b        A  0.94286900
6       6        c        A -0.01325556
df<- d