Adding an extra point in a ggplot2 graph

Using annotate:

ggplot(iris, aes(x = Sepal.Width, y = Sepal.Length, col = Species)) + 
  geom_point() +
  annotate("point", x = 5.6, y = 3.9, colour = "blue")

library('ggplot2')

df = data.frame(Sepal.Width = 5.6, Sepal.Length = 3.9) 

ggplot(iris, aes(x = Sepal.Width, y = Sepal.Length, col = Species)) +
  geom_point() +
  geom_point(data = df, col = 'blue')

Add another layer:

ggplot(iris, aes(x = Sepal.Width, y = Sepal.Length, col = Species)) + 
  geom_point() +
  geom_point(aes(x=5.6, y=3.9), colour="blue")