如何将ggplot scale_color_manual与可变值一起使用

Question

以下情节（从here借来的示例）在cut和scale_color_manual中使用了硬编码的间隔，并且运行良好：

ggplot(mtcars, aes(wt, mpg)) + 
  geom_point(aes(colour = cut(qsec, c(-Inf, 17, 19, Inf))),
             size = 5) +
  scale_color_manual(name = "qsec",
                     values = c("(-Inf,17]" = "black",
                                "(17,19]" = "yellow",
                                "(19, Inf]" = "red"),
                     labels = c("<= 17", "17 < qsec <= 19", "> 19"))

但是，我想使用一个变量来定义这些范围，从而导致图形上没有绘制任何图形：

low <- 17
high <- 19

ggplot(mtcars, aes(wt, mpg)) + 
  geom_point(aes(colour = cut(qsec, c(-Inf, low, high, Inf))),
             size = 5) +
  scale_color_manual(name = "qsec",
                     values = c("(-Inf,low]" = "black",
                                "(low,high]" = "yellow",
                                "(high, Inf]" = "red"),
                     labels = c("<= 17", "17 < qsec <= 19", "> 19"))

我知道问题在于scale_color_manual是如何解释变量的，因为以下方法可以工作（但不允许我定义颜色）：

low <- 17
high <- 19

ggplot(mtcars, aes(wt, mpg)) +
  geom_point(aes(colour = cut(qsec, c(-Inf, low, high, Inf))),
             size = 5)

为什么不起作用？