在stat_contour中设置特定于刻面的中断

我想使用ggplot和stat_contour为的两类数据显示等高线图facet_grid。我想根据数据突出显示特定级别。这是一个使用常规volcano数据的类似虚拟示例。

library(dplyr)
library(ggplot2)    
v.plot <- volcano %>% reshape2::melt(.) %>% 
      mutate(dummy = Var1 > median(Var1)) %>% 
      ggplot(aes(Var1, Var2, z = value)) + 
      stat_contour(breaks = seq(90, 200, 12)) + 
      facet_grid(~dummy)

情节1：

假设在每个因子水平（我想是东半和西半）内，我想找到火山的平均高度并将其显示出来。我可以手动计算：

volcano %>% reshape2::melt(.) %>% 
  mutate(dummy = Var1 > median(Var1)) %>% 
  group_by(dummy) %>% 
  summarise(h.bar = mean(value))

# A tibble: 2 × 2
  dummy    h.bar
  <lgl>    <dbl>
1 FALSE 140.7582
2  TRUE 119.3717

这告诉我，每半部分的平均高度分别为141和119。我可以在两个面上都画出这两个高度，而不仅仅是在每侧上画出适当的高度。

v.plot + stat_contour(breaks = c(141, 119), colour = "red", size = 2)

情节2：

And you can't put breaks= inside an aes() statement, so passing it in as a column in the original dataframe is out. I realize with this dummy example I could probably just do something like bins=2 but in my actual data I don't want the mean of the data, I want something else altogether.

Thanks!