Often you may want to add a manual legend to a plot in ggplot2 with custom colors, labels, title, etc.

Fortunately this is simple to do using the scale_color_manual() function and the following example shows how to do so.

Example: Create Manual Legend in ggplot2

The following code shows how to plot three fitted regression lines in a plot in ggplot2 with a custom manual legend:

library(ggplot2)

#create data frame
df <- data.frame(x=c(1, 2, 2, 3, 5, 6, 8, 8, 9, 9, 10, 11, 12, 15, 15),
                 y=c(2, 3, 3, 4, 5, 5, 6, 7, 8, 8, 9, 10, 16, 19, 28))

#create plot with three fitted regression models
ggplot(df, aes(x, y)) +
  geom_point() +
  geom_smooth(se=FALSE, aes(color='Linear')) +
  geom_smooth(formula=y~poly(x, 2), se=FALSE, aes(color='Quadratic')) +
  geom_smooth(formula=y~poly(x, 3), se=FALSE, aes(color='Cubic')) +
  scale_color_manual(name='Regression Model',
                     breaks=c('Linear', 'Quadratic', 'Cubic'),
                     values=c('Cubic'='pink', 'Quadratic'='blue', 'Linear'='purple'))

Using the scale_color_manual() function, we were able to specify the following aspects of the legend:

• name: The title of the legend
• breaks: The labels in the legend
• values: The colors in the legend

Note that we can also use the theme() function to modify the font size of the elements in the legend:

library(ggplot2)

#create data frame
df <- data.frame(x=c(1, 2, 2, 3, 5, 6, 8, 8, 9, 9, 10, 11, 12, 15, 15),
                 y=c(2, 3, 3, 4, 5, 5, 6, 7, 8, 8, 9, 10, 16, 19, 28))

#create plot with three fitted regression models
ggplot(df, aes(x, y)) +
  geom_point() +
  geom_smooth(se=FALSE, aes(color='Linear')) +
  geom_smooth(formula=y~poly(x, 2), se=FALSE, aes(color='Quadratic')) +
  geom_smooth(formula=y~poly(x, 3), se=FALSE, aes(color='Cubic')) +
  scale_color_manual(name='Regression Model',
                     breaks=c('Linear', 'Quadratic', 'Cubic'),
                     values=c('Cubic'='pink', 'Quadratic'='blue', 'Linear'='purple'))+
 theme(legend.title=element_text(size=20),
       legend.text=element_text(size=14))

Notice that the font size of both the title and the labels in the legend were increased.

The following tutorials explain how to perform other common operations in ggplot2: