How to draw a function on a scatter plot in ggplot

Question

I want to plot a function -- eg a line-- inside a scatterplot. I have composed code that do both separately but how I can combine them? My experimentations returned error messages.

My code is the following:

library(ROSE)
data(hacide)
train <- hacide.train

Scatter plot

ggplot(train, aes(x1, x2, colour = cls)) +
  geom_point(size = 3, alpha = 0.4)

Line

db <- function(x, beta1, beta2, alpha){-alpha/beta2 - x * beta1/beta2}

ggplot(data.frame(x = c(-4, 4)), aes(x = x))  +
  stat_function(fun = db, args = list(-1.642354, -1.596056, -6.004609), colour = "blue" , size = 2)

But how to superimpose / combine the line with the scatter plot?

Answer 1

Is that what you are looking for ?

ggplot(data.frame(x = c(-4, 4)), aes(x = x))  +
  stat_function(fun = db, args = list(-1.642354, -1.596056, -6.004609), colour = "blue" , size = 2)+
  geom_point(data = train, aes(x1, x2, colour = cls),  size = 3, alpha = 0.4)

Answer 2

The most simple way to do this is just add the function to the original plot. The graphs in ggplot2 are constructed in layers, meaning you can always add more layers while you construct your graphs.

For your code you could either do:

library(ROSE)
data(hacide)
train <- hacide.train

db <- function(x, beta1, beta2, alpha){-alpha/beta2 - x * beta1/beta2}

ggplot(train, aes(x1, x2, colour = cls)) +
  geom_point(size = 3, alpha = 0.4) +
  stat_function(fun = db, args = list(-1.642354, -1.596056, -6.004609),
  colour = "blue" , size = 2)

or add it to the existing plot:

library(ROSE)
data(hacide)
train <- hacide.train

plot = ggplot(train, aes(x1, x2, colour = cls)) +
       geom_point(size = 3, alpha = 0.4)

db <- function(x, beta1, beta2, alpha){-alpha/beta2 - x * beta1/beta2}

plot + stat_function(fun = db, args = list(-1.642354, -1.596056, -6.004609),
colour = "blue" , size = 2)