Setting facet-specific breaks in stat_contour

Question

I'd like to show a contour plot using ggplot and stat_contour for two categories of my data with facet_grid. I want to highlight a particular level based on the data. Here's an analogous dummy example using the usual volcano data.

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)

Plot 1:

Let's say within each factor level (here east and west halves, I guess), I want to find the mean height of the volcano and show that. I can calculate it manually:

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

Which tells me that the mean heights on each half are 141 and 119. I can draw BOTH of those on BOTH facets, but not just the appropriate one on each side.

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

Plot 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!

Answer 1

I made another attempt at this problem and came up with a partial solution, but I'm forced to use a different geom.

volcano %>% reshape2::melt(.) %>% 
  mutate(dummy = Var1 > median(Var1)) %>% 
  group_by(dummy) %>% 
  mutate(h.bar = mean(value),                      # edit1
         is.close = round(h.bar) == value) %>%     #
  ggplot(aes(Var1, Var2, z = value)) + 
  stat_contour(breaks = seq(90, 200, 12)) + 
  geom_point(colour = "red", size = 3,             # edit 2
           aes(alpha = is.close)) +                #
  scale_alpha_discrete(range = c(0,1)) +           #
  facet_grid(~dummy)

In edit 1 I added a mutate() to the above block to generate a variable identifying where value was "close enough" (rounded to the nearest integer) to the desired highlight point (the mean of the data for this example).

In edit2 I added geom_points to show the grid locations with the desired value, and hid the undesired ones using an alpha of 0 or totally transparent.

Plot 3:

The problem with this solution is that it's very gappy, and trying to bridge those with geom_path is a jumbled mess. I tried coarser rounding as well, and it just made things muddy.

Would love to hear other ideas! Thanks