How to pipe forward a ggplot object?

Question

I would like to use a custom function defined on a ggplot object with magrittr's pipe. However, I cannot pipe a ggplot object into this function.

Here is a simple example:

library(ggplot2)
library(magrittr)

my_plot_function <- function(plot) {
    plot + geom_hline(yintercept = 3, linetype = 'dashed')
}

data(mtcars)
p <- mtcars %>%
    ggplot() +
    geom_point(aes(mpg, wt))
my_plot_function(p)

It would be great if I could use my_plot_function() within the chain as follows:

mtcars %>%
    ggplot() +
    geom_point(aes(mpg, wt)) %>%
    my_plot_function()

However, it gives an error as only the layer is passed to my_plot_function() instead of the plot itself. How could I pass the plot with the pipe?

Answer 1

You can try to define a function that does not expect a plot object and just add it as is usual in ggplot.

my_plot_function <- function() {
    geom_hline(yintercept = 3, linetype = 'dashed')
}


mtcars %>%
    ggplot() +
    geom_point(aes(mpg, wt)) + my_plot_function()