adding a ggplot2 stat_function to an existing graph

Question

I've got a graph and I'd like to superimpose a functional form over the graph. Seems straight forward, but I'm missing something. Here's my example data and illustration where I use stat_function to try and add f1 to the graph:

library(ggplot2)
sample_data <- data.frame(rate  = c(0.514492753623188, 0.553072625698324, 0.656527249683143, 0.675694939415538,
                                    0.68681076312307, 0.715657311669128, 0.792349726775956), 
                          level = c(0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85) )

f1 <- function(x) pbeta(x, shape1=1.01, shape2=.9 ) * 100

g <- ggplot() +
  geom_line(aes(y = rate*100 , x = level), 
            data = sample_data, stat="identity") +
  stat_function(fun = f1) 
g

As you can see, the f1 line is not there.

So as an alternative, I can add the points to the initial data.frame and then plot them with geom_line as the following example illustrates:

## add f1 to data.frame
sample_data$f1 <- f1(sample_data$level )

g <- ggplot() +
  geom_line(aes(y = rate*100 , x = level), 
            data = sample_data, stat="identity") +
  geom_line(aes(y = f1 , x = level), 
   data = sample_data, stat="identity", color='red')
g

So that works, and gets me what I need, but I can't figure out why stat_function didn't work. Any tips?

Answer 1

You need data = sample_data, aes(y = rate*100 , x = level) in your first ggplot call so stat_function knows about the data are being used

library(ggplot2)

sample_data <- data.frame(rate  = c(0.514492753623188, 0.553072625698324, 0.656527249683143, 0.675694939415538,
                                    0.68681076312307, 0.715657311669128, 0.792349726775956), 
                          level = c(0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85) )

f1 <- function(x) pbeta(x, shape1=1.01, shape2=.9 ) * 100

g <- ggplot(data = sample_data, aes(y = rate*100 , x = level)) +
  geom_line() +
  stat_function(fun = f1, color = "red")
g

Created on 2018-05-20 by the reprex package (v0.2.0).