Add legend to boxplot rstudio

Question

I use the following code to create a boxplot:

plot <- ggplot(WL, aes(y = wavelength, x = factor(category, level = c("A", "B")))) +
  theme_bw() +
  geom_boxplot(outlier.colour = "gray30", outlier.shape = 8, outlier.size = 2, lwd=1, fill = c("#C6DBEF", "#FEE391")) +
  ylab(expression(lambda[(km)])) +
  theme(plot.margin = unit(c(2,2,2,2), "cm"),
        axis.title.x = element_blank(),
        axis.title.y=element_text(size=20),
        plot.title = element_text(size = 22, vjust = 2)) 

How can I add a legend? I want to place it inside the plot (topright). It should show the two colors and a description.

I am trying to use legends() which does not work for me.

Answer 1

The issue is where you specified the fill - The following code will give you a legend, and changing the position in the last line will allow you to change it where you want:

# sample data
WL <- data.frame(wavelength = rnorm(100, 0,1),
                 category = sample(LETTERS[1:2], 100, replace = TRUE))

# plot
ggplot(WL, aes(y = wavelength, x = factor(category, level = c("A", "B")), fill = category)) +
  theme_bw() +
  geom_boxplot(outlier.colour = "gray30", outlier.shape = 8, outlier.size = 2, lwd = 1) +
  ylab(expression(lambda[(km)])) +
  theme(plot.margin = unit(c(2,2,2,2), "cm"),
        axis.title.x = element_blank(),
        axis.title.y=element_text(size=20),
        plot.title = element_text(size = 22, vjust = 2)) +
  scale_fill_manual(values = c("#C6DBEF", "#FEE391")) + 
  theme(legend.position = "bottom")