Creating Separate Legend for Specific Line in ggplot2

I'm working on a data visualization project in R using the ggplot2 package. I have a bar plot with two bars per year and a trend line that applies to one of the bars. I'm trying to create a clear legend for the plot. Currently, both bars have a dashed line in the legend instead of just one, and the legend entries are colored squares with the dashed line.

Here's the code I have so far:

# Creating a data frame
year <- c(2013:2022)
procent <- c(80, 75, 70, 65, 60, 55, 60, 50, 55, 50)
group <- "GroupA" 
DataA <- data.frame(Year = year, Procent = procent, Group = group)

year <- c(2013:2022)
procent <- c(20, 25, 30, 35, 40, 45, 40, 50, 45, 50)
group <- "GroupB"
DataB <- data.frame(Year = year, Procent = procent, Group = group)

Data <- rbind(DataA,DataB)

# Plotting the data
ggkrs <- ggplot(data = Data, aes(x = Year, y = Procent, fill = Group)) + 
  geom_bar(stat = "identity", position = "dodge", width = 0.6, size = 0.1) +
  geom_smooth(data = . %>% filter(Group == "GroupB"), method = lm, aes(x = Year, y = Procent), se = FALSE, linetype = "dashed", color = "red") +
  labs(title = "Development of Buyer Flows") +
  scale_fill_manual(values = c("GroupA" = "blue", "GroupB" = "green")) +
  scale_x_continuous(breaks = seq(min(year), max(year), by = 1)) +
  scale_y_continuous(breaks = seq(0, 100, by = 10), limits = c(0, 90)) +
  scale_linetype_manual(values = c("dashed"), name = "Trendline") +
  theme_minimal() +
  theme(legend.position = "bottom", legend.title = element_blank())

print(ggkrs)

I've tried different approaches, including separating the data for the trend line and using scale_linetype_manual(), but I haven't been able to achieve the desired result.

I've seen similar legends in Excel, where a separate legend entry for a specific line is shown. Is there a way to achieve this in ggplot2? Any guidance or examples would be greatly appreciated.

Thank you in advance for your help!

Solution

One potential option is to move color into your aes() for geom_smooth(). You can then name the trendline legend entry whatever you like, e.g.

library(tidyverse)

# Creating a data frame
year <- c(2013:2022)
procent <- c(80, 75, 70, 65, 60, 55, 60, 50, 55, 50)
group <- "GroupA" 
DataA <- data.frame(Year = year, Procent = procent, Group = group)

year <- c(2013:2022)
procent <- c(20, 25, 30, 35, 40, 45, 40, 50, 45, 50)
group <- "GroupB"
DataB <- data.frame(Year = year, Procent = procent, Group = group)

Data <- rbind(DataA,DataB)

# Plotting the data
ggkrs <- ggplot(data = Data, aes(x = Year, y = Procent, fill = Group)) + 
  geom_bar(stat = "identity", position = "dodge", width = 0.6, size = 0.1) +
  geom_smooth(data = . %>% filter(Group == "GroupB"), 
              method = lm, aes(x = Year, y = Procent, color = "trendline of Procent per Year"), 
              se = FALSE, linetype = "dashed", inherit.aes = FALSE) +
  labs(title = "Development of Buyer Flows") +
  scale_fill_manual(values = c("GroupA" = "blue", "GroupB" = "green")) +
  scale_x_continuous(breaks = seq(min(year), max(year), by = 1)) +
  scale_y_continuous(breaks = seq(0, 100, by = 10), limits = c(0, 90)) +
  scale_linetype_manual(values = c("dashed"), name = "Trendline") +
  theme_minimal() +
  theme(legend.position = "bottom", legend.title = element_blank())
#> Warning: Using `size` aesthetic for lines was deprecated in ggplot2 3.4.0.
#> ℹ Please use `linewidth` instead.
#> This warning is displayed once every 8 hours.
#> Call `lifecycle::last_lifecycle_warnings()` to see where this warning was
#> generated.

print(ggkrs)
#> `geom_smooth()` using formula = 'y ~ x'

^{Created on 2023-08-30 with reprex v2.0.2}