Adding an additional legend manually for different data.frames used in the same ggplot

Question

In my plot below, I have two separate sources of data (dat and dat2) used in two different geom_smooth() calls producing the black and the red regression lines (see pic below).

Is it possible to manually add another legend that shows the black line is called "Between" and red line is called "Within"?

library(tidyverse)

dat <- read.csv('https://raw.githubusercontent.com/rnorouzian/e/master/cw2.csv')
dat$groups <- factor(dat$groups)

dat2 <- dat %>% group_by(groups) %>% summarize(mean_x = mean(x),
                                               mean_y = mean(y),
                                               .groups = 'drop')
dat %>% 
  ggplot() +
  aes(x, y, color = groups, shape = groups)+
  geom_point(size = 2) + theme_classic()+ 
  stat_ellipse(level = .6) +
  geom_point(data = dat2, 
             mapping = aes(x = mean_x, y = mean_y,fill = factor(groups)),
             size = 4, show.legend = F,shape=21) +
  geom_smooth(data = dat2, mapping = aes(x = mean_x, y = mean_y,group=1), 
          method = "lm", se=F, color = 1, formula = 'y ~ x')+ 
  geom_smooth(aes(group = 1), 
              method = "lm", se=F, color = 2, formula = 'y ~ x')+
  scale_fill_manual(values=rep('black',3))

Answer 1

It looks like you need a second color scale to do this. You can use the ggnewscale package:

library(ggnewscale)

dat %>% 
  ggplot() +
  aes(x, y, color = groups, shape = groups) +
  geom_point(size = 2) + 
  theme_classic() + 
  stat_ellipse(level = .6) +
  geom_point(data = dat2, 
             mapping = aes(x = mean_x, y = mean_y),
             size = 4, show.legend = FALSE, shape = 21, fill = "black") +
  scale_color_discrete() +
  new_scale_color() +
  geom_smooth(data = dat2, 
              mapping = aes(x = mean_x, y = mean_y, group = 1, color = "black"), 
          method = "lm", se = FALSE, formula = 'y ~ x') + 
  geom_smooth(aes(group = 1, color = "red"), 
              method = "lm", se = FALSE, formula = 'y ~ x') +
  scale_color_identity(name = "", labels = c("Between", "Within"),
                       guide = guide_legend())