How can an S3 plot theme be utilized to apply a theme to several plots through a custom user function?

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An S3 plot theme can be utilized to apply a theme to several plots through a custom user function as follows:

1. Define the plot theme as an S3 class by creating a new plot function that extends the ggplot2 function and returns a modified ggplot2 object with the desired theme. For example:

my_theme <- function(...) {
  ggplot2::ggplot(...) +
    ggplot2::theme_minimal() +
    ggplot2::theme(legend.position = "bottom")
}

class(my_theme) <- c("my_theme", "ggplot")

1. Create a custom user function that applies the plot theme to a given plot object. This function should take a plot object as input, modify it using the ggplot2::ggplot_build and ggplot2::ggplot_gtable functions, and return the modified plot object with the theme applied. For example:

apply_theme <- function(plot) {
  plot_data <- ggplot2::ggplot_build(plot)
  plot_table <- ggplot2::ggplot_gtable(plot_data)
  plot_table$add_theme(my_theme())
  grid::grid.draw(plot_table)
  plot
}

1. Call the custom user function with each plot object that you want to apply the theme to. For example:

plot1 <- ggplot2::ggplot(mtcars, aes(mpg, wt)) + geom_point()
plot2 <- ggplot2::ggplot(iris, aes(Sepal.Length, Sepal.Width, color = Species)) + geom_point()

plot1_with_theme <- apply_theme(plot1)
plot2_with_theme <- apply_theme(plot2)

This will apply the my_theme plot theme to plot1 and plot2, and return modified plot objects with the theme applied.