How to use a different fill gradient based on value threshold in ggplot2?

Data

dat <- structure(list(Date = structure(c(19913, 19913, 19913, 19913, 
19914, 19914, 19914, 19914, 19915, 19915, 19915, 19915, 19916, 
19916, 19916, 19916), class = "Date"), meal = structure(c(1L, 
2L, 3L, 4L, 1L, 2L, 3L, 4L, 1L, 2L, 3L, 4L, 1L, 2L, 3L, 4L), levels = c("Fasting", 
"Breakfast", "Lunch", "Dinner"), class = "factor"), value = c(NA, 
NA, 121, 136, 75, 82, 86, 114, 80, 81, 78, 90, 75, 91, NA, NA
), threshold = c(NA, NA, FALSE, FALSE, TRUE, TRUE, TRUE, TRUE, 
TRUE, TRUE, TRUE, TRUE, TRUE, TRUE, NA, NA)), row.names = c(NA, 
-16L), class = "data.frame")

   Date       meal      value threshold
   <date>     <fct>     <dbl> <lgl>    
 1 2024-07-09 Fasting      NA NA       
 2 2024-07-09 Breakfast    NA NA       
 3 2024-07-09 Lunch       121 FALSE       
 4 2024-07-09 Dinner      136 FALSE    
 5 2024-07-10 Fasting      75 TRUE     
 6 2024-07-10 Breakfast    82 TRUE     
 7 2024-07-10 Lunch        86 TRUE     
 8 2024-07-10 Dinner      114 TRUE     
 9 2024-07-11 Fasting      80 TRUE     
10 2024-07-11 Breakfast    81 TRUE     
11 2024-07-11 Lunch        78 TRUE     
12 2024-07-11 Dinner       90 TRUE     
13 2024-07-12 Fasting      75 TRUE     
14 2024-07-12 Breakfast    91 TRUE     
15 2024-07-12 Lunch        NA NA       
16 2024-07-12 Dinner       NA NA       

Problem

I want to create a tile plot with two different gradients based on a threshold:

1. Where threshold is FALSE the higher the value the more red
2. Where threshold is TRUE the lower the value the more green
3. Where threshold is NA grey is fine (this is the default anyway).

Attempt

library(ggplot2) 

ggplot(dat, aes(x = meal, y = Date, fill = threshold)) +
  geom_tile() +
  geom_text(aes(label = value), color = "white", fontface = "bold") +
  scale_fill_manual(values = c("#ef5350", "#48bf53")) +
  scale_x_discrete(expand = c(0, 0)) +
  scale_y_date(expand = c(0, 0), date_breaks = "1 day", date_labels = "%B %d") +
  theme(legend.position = "none") +
  labs(x = element_blank(), y = element_blank())

This output is what I would like but I cannot get the gradient to work within threshold.

>Solution :

The usual trick to do this, is to map value to the alpha transparency. In your case, you just need to flip the direction for the green case first:

library(ggplot2) 

dat |> 
  mutate(
    alpha_scale = ifelse(threshold, scales::rescale(-value), scales::rescale(value)),
    .by = threshold
  ) |> 
  ggplot(aes(x = meal, y = Date, fill = threshold)) +
  geom_tile(aes(alpha = alpha_scale)) +
  geom_text(aes(label = value), color = "white", fontface = "bold") +
  scale_fill_manual(values = c("#ef5350", "#48bf53")) +
  scale_x_discrete(expand = c(0, 0)) +
  scale_y_date(expand = c(0, 0), date_breaks = "1 day", date_labels = "%B %d") +
  theme(legend.position = "none") +
  labs(x = element_blank(), y = element_blank())