How to write a function to add multiple ggproto objects to a ggplot

ggplot_function_reprex.R

#' ---
#' output:
#'   md_document:
#'     pandoc_args:
#'       - '--from=markdown-implicit_figures'
#'       - '--to=commonmark'
#'       - '--wrap=preserve'
#' ---



#+ reprex-setup, include = FALSE
options(tidyverse.quiet = TRUE)
knitr::opts_chunk$set(collapse = TRUE, comment = "#>", error = TRUE)
knitr::opts_knit$set(upload.fun = knitr::imgur_upload)

#+ reprex-body
library(tidyverse)
dat1 <- tibble(a = 1:3, b = 1:3)
dat2 <- tibble(l = c(2))

plot_hline1 <- function(ggplot_obj, yint, dat = dat2) {
  ggplot_obj +
    geom_hline(data = dat, aes(yintercept = {{ yint }})) +
    theme_classic()
}
plot_hline2 <- function(yint, dat = dat2) {
  geom_hline(data = dat, aes(yintercept = {{ yint }})) + 
    theme_classic()
}
plot_hline3 <- function(yint, dat = dat2) {
  list(geom_hline(data = dat, aes(yintercept = {{ yint }})),
       theme_classic()
  )
}

# 🚫 fails with pipe to pass the ggplot object
ggplot(dat1, aes(a, b)) +
  geom_point() %>% 
  plot_hline1(yint = l)

# 🚫 fails adding ggproto objects
ggplot(dat1, aes(a, b)) +
  geom_point() + 
  plot_hline2(yint = l)

# ✅ works when you use a list of ggproto objects
ggplot(dat1, aes(a, b)) +
  geom_point() + 
  plot_hline3(yint = l)



#' <sup>Created on `r Sys.Date()` by the [reprex package](https://reprex.tidyverse.org) (v`r utils::packageVersion("reprex")`)</sup>