在格子堆栈条形图

Question

是否有一种方法可以合并不同的lattice barchart图来获得堆叠的barchart？ （我可以使用as.layer，但是这两个图是彼此相对而不是堆叠的。例如：

library(lattice); library(dplyr); library(latticeExtra)

# sample data
barley %>% filter(variety %in% c("Trebi","Velvet","Peatland"))

b1   <- barley %>% filter(site == "Morris")
b2   <- barley %>% filter(site == "Duluth")
b1a2 <- barley %>% filter(site %in% c("Morris","Duluth"))

# stacked plot
plotorg <- barchart(yield ~ variety | year, data = b1a2,
                    groups = site, stack = TRUE)

# two barcharts that I want stack together
plot1   <- barchart(yield ~ variety | year, data = b1,
                    groups = site, stack = TRUE)

plot2   <- barchart(yield ~ variety | year, data = b2,
                    groups = site, stack = TRUE)

plot1 + plot2 # bars are behind each other and not stacked