如何在 ggplot2 R 中为多面数据指定多个 xlim？

数据由两个变量组成（见图）。每个变量都有不同的范围。我想指定范围，以便 var1 和 vae2 中的所有图都受这些变量的最小值和最大值的约束。请参阅随附的示例代码。我不想setscales = "free"在 facet_wrap 上使用。

var1 <- rnorm(100, 6, 2)
var2 <- rnorm(100,15,2)
spp.val <- rnorm(100,10,2)
spp <- rep(c("A","B","C","D"), 25)

df <- data.frame(var1, var2,spp, spp.val)

df <- gather(df,
key = "var",
value = "var.val",
var1,var2)

df$var <- as.factor(as.character(df$var))
df$spp <- as.factor(as.character(df$spp))

ggplot(aes(x = var.val, y = spp.val), data = df) +
  geom_point() +
  facet_grid(spp~var)

#I want the limits for each facet_grid to be set as follows
xlim(min(df[df$var == "var1",]), max(df[df$var == "var1",])
xlim(min(df[df$var == "var2",]), max(df[df$var == "var2",])