I would like to plot densities by groups such that the alpha value decreases (more transparent) as the x axis value gets closer to 0.

Based on the data dataset, I generate the alpha column by rescaling the x axis values around 0.

I thought that adding the alpha inside the aes() would work but this throws and error.

library(tidyverse)
library(purrr)
library(scales)

set.seed(123)
data <- tibble(A = rnorm(100),
               B = rnorm(100, mean = -0.7),
               C = rnorm(100, mean = 1)) %>% 
  pivot_longer(cols = everything(), 
               names_to = "model") %>% 
  group_by(model) %>% 
  summarise(value = list(value)) %>%
  mutate( xval = map(value, ~density(.x)$x),
          yval = map(value, ~density(.x)$y)) %>% 
  select(-value) %>% 
  unnest(ends_with("val"))

#create alpha column
df <- data %>% 
  group_by(model) %>% 
  mutate(myalpha = abs(scale(xval, center = 0)), #scale to center around 0
         myalpha2 = scales::rescale_mid(myalpha, mid = 0) #rescale 0-1, 0 for values around 0
         ) %>% 
  as_tibble() 

df %>% 
  ggplot(aes(x = xval, y = yval,
             fill = model, col = model)) 
  geom_line() 
  geom_vline(xintercept = 0) 
  geom_density(aes(alpha = myalpha2), #alpha white around 0
               stat = "identity") 
  scale_fill_manual(values = c("red", "pink", "orange")) 
  scale_alpha_identity()
#> Error in `f()`:
#> ! Aesthetics can not vary with a ribbon

^{Created on 2022-09-11 by the}