I have a time series for two groups. I am using stat_smooth to display the linear relationship for the two groups on a time series plot. I want the line of best fit to be solid for one group and dashed for the other group. I can get close to what I want, please see below, but my attempts either 1) assigns the line type to the wrong group or 2) creates two legends which I want to avoid. How can I assign a specific line type to a specific group when using stat_smooth?

library(tidyverse) 

# Example time series

set.seed(222)

df <- data.frame(
  date = rep(seq(as.Date('2024-04-01'),
                 as.Date('2024-04-05'),
                 '1 day'), times = 2),
  group = c('a','a','a','a','a',
            'z','z','z','z','z'),
  value = rnorm(10, 1, 2)
)

# Figure 1 (both groups same line type)

df %>%
  ggplot(aes(x = date, y = value, group = group)) +
  geom_line(aes(col = group)) +
  geom_point(aes(col = group), shape = 16) +
  stat_smooth(method = 'lm', se = F, color = 'black') +
  theme_bw()
#> `geom_smooth()` using formula = 'y ~ x'

# Figure 2 (line type changed but assigned to wrong groups)

df %>%
  ggplot(aes(x = date, y = value, group = group, linetype = group)) +
  geom_line(aes(col = group), linetype = 'solid') +
  geom_point(aes(col = group), shape = 16) +
  stat_smooth(method = 'lm', se = F, color = 'black') +
  theme_bw()
#> `geom_smooth()` using formula = 'y ~ x'

# Figure 3 (line type assigned to correct groups but now a second legend appears)

df %>%
  ggplot(aes(x = date, y = value, group = group, linetype = fct_rev(group))) + # NOTE same thing happend when using rev()
  geom_line(aes(col = group), linetype = 'solid') +
  geom_point(aes(col = group), shape = 16) +
  stat_smooth(method = 'lm', se = F, color = 'black') +
  theme_bw()
#> `geom_smooth()` using formula = 'y ~ x'

^{Created on 2024-05-15 with reprex v2.1.0}