Fill area between two lines, with high/low and dates

Question

Forword: I provide a reasonably satisfactory answer to my own question. I understand this is acceptable practice. Naturally my hope is to invite suggestions and improvements.

Answer 1

@walts answer should remain the winner but while implementing his solution, I gave it a tidy update.

library(tidyverse)
set.seed(2345)

# fake data​
raw_data <-
  tibble(
    date = as.Date("2020-01-01") + (1:40),
    a = 95 + cumsum(runif(40, min = -20, max = 20)),
    b = 55  + cumsum(runif(40, min = -1, max = 1))
  )
​
# the steps
# the 'y' + 'min_line'  + 'group' is the right granularity (by date) to 
# create 2 separate ribbons
df <-
  raw_data %>% 
  # find min of the two columns
  mutate(min_line = pmin(a, b)) %>% 
  pivot_longer(c(a, b), names_to = "group", values_to = "y") %>% 
  print()
​
# the result
ggplot(data = df, aes(x = date, fill = group)) +
  geom_ribbon(aes(ymax = y, ymin = min_line)) +
  theme_classic()