Вы можете попробовать это:
library(dplyr)
library(ggplot2)
df %>%
group_by(Customer, Server) %>%
summarise(Size = sum(Size)) %>%
ggplot() + aes(x = Server, y = Size, fill = Customer) + geom_col()
data
df <- structure(list(Customer = structure(c(1L, 1L, 1L, 2L, 2L, 2L,
3L, 3L, 3L), .Label = c("Cus_a", "Cus_b", "Cus_c"), class = "factor"),
Server = structure(c(1L, 1L, 2L, 2L, 2L, 2L, 2L, 2L, 3L), .Label = c("Ser_1",
"Ser_2", "Ser_3"), class = "factor"), Size = c(3L, 4L, 2L,
1L, 3L, 2L, 4L, 1L, 4L)), class = "data.frame", row.names = c(NA,-9L))