R: legend not showing when using ggplot

Question

I'd like to include a legend for a plot. However, nothing shows up. I've looked at other similar questions but none seem to solve my case.

Here's that section of the code.

ggplot(lm_model, aes(x=year, y=pred_price)) +
  geom_point(color="red") +
  geom_line(color="red") +
  geom_line(aes(x=year, y=real_price)) +
  labs(title="Linear Regression",
       x="Year",
       y="Gas Price") +
  scale_color_manual(labels = c("Predicted", "True Value"))

Here's how the output looks with this code (and without the legend as you can see):

Answer 1

This can work but not tested as no data was shared. You need to move the color statements inside aes():

library(ggplot2)
#Data
lm_model <- data.frame(year=2010:2020,
                       pred_price=runif(11,0,75),
                       real_price=runif(11,0,75))
#Code
ggplot(lm_model, aes(x=year, y=pred_price)) +
  geom_point(aes(color="red")) +
  geom_line(aes(color="red")) +
  geom_line(aes(x=year, y=real_price,color='black')) +
  labs(title="Linear Regression",
       x="Year",
       y="Gas Price") +
  scale_color_manual(labels = c("Predicted", "True Value"),
                     values=c('black','red'))+
  labs(color='Price')

Output: