ggplot legend does not work

Hi, I have a following problem. I plot two lines using ggplot package - density plot and standard curve. See my code bellow:

ggplot(, aes(x=resid(RegreseT) )) + 
  geom_density(alpha=.2 , color = "darkblue") +
  stat_function(fun = dnorm, args = list(mean = mean(resid(RegreseT)), sd = sd(resid(RegreseT))) , colour = "red") +
  xlab("Residuals of Temperature ~ Year_Month")+ylab("Density") +
  labs(colour="I need a legend!!!!") +
  theme(legend.key.height=unit(2, "cm"))

However, there is no legend in the output. Why, please? Thanks a lot.

To get a legend, the color should be set via the aes() function passed to the mapping argument of a geom. Here is an example of manually setting the colors using data that I think are similar to what you have.

DF <- data.frame(Resid = rnorm(100))
COLORS <- c("Data" = "darkblue", "FIT" = "red")
ggplot(DF, aes(x= Resid)) + 
  geom_density(alpha=.2 , mapping = aes(color = "Data")) +
  stat_function(fun = dnorm, 
                args = list(mean = mean(DF$Resid), sd = sd(DF$Resid)), 
                mapping = aes(color = "FIT")) +
  xlab("Residuals of Temperature ~ Year_Month")+ylab("Density") +
  #labs(colour="I need a legend!!!!") +
  scale_color_manual(values = COLORS) +
  theme(legend.key.height=unit(2, "cm"))

Created on 2020-08-20 by the reprex package (v0.3.0)

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