if else condition in ggplot to add an extra layer

Question

say I want to plot two layers in ggplot, one containing points and another one containing lines if a certain criteria is fulfilled.

The code without the criteria cou

Answer 1

Following the ggplot2 book, you can create a function which returns a list. Any NULL components will be ignored.

library(ggplot2)
library(ggplot2movies)

# Summarise number of movie ratings by year of movie
mry <- do.call(rbind, by(movies, round(movies$rating), function(df) {
    nums <- tapply(df$length, df$year, length)
    data.frame(rating=round(df$rating[1]), year = as.numeric(names(nums)), number=as.vector(nums))
}))

# create function to add points conditionally
# If the list contains any NULL elements, they’re ignored. 
my_plot <- function(point = FALSE){
    list(
        geom_line(),
        if (point) 
            geom_point()
    )

}

p <- ggplot(mry, aes(x=year, y=number, group=rating))
p + my_plot()

p + my_plot(point = TRUE)

^{Created on 2020-02-25 by the reprex package (v0.3.0)}