19.7 The gtable step

Again, still working with our plot p

p <- ggplot(mpg, aes(displ, hwy, color = drv)) + 
  geom_point(position = position_jitter(seed = 2022)) +
  geom_smooth(method = "lm", formula = y ~ x) + 
  facet_wrap(vars(year)) + 
  ggtitle("A plot for expository purposes")

p

# print(p)
# plot(p)

The return value of ggplot_build() contains the computed data associated with each layer and a Layout ggproto object which holds information about data other than the layers, including the scales, coordinate system, facets, etc.

names(ggplot_build(p))

[1] "data"   "layout" "plot"  

Layout data

ggplot_build(p)$data %>% purrr::map(head, 3)

class(ggplot_build(p)$layout)

[1] "Layout"  "ggproto" "gg"     

The output of ggplot_build() is then passed to ggplot_gtable() to be converted into graphical elements before being drawn:

ggplot2:::plot.ggplot

function (x, newpage = is.null(vp), vp = NULL, ...) 
{
    set_last_plot(x)
    if (newpage) 
        grid.newpage()
    grDevices::recordGraphics(requireNamespace("ggplot2", quietly = TRUE), 
        list(), getNamespace("ggplot2"))
    data <- ggplot_build(x)
    gtable <- ggplot_gtable(data)
    if (is.null(vp)) {
        grid.draw(gtable)
    }
    else {
        if (is.character(vp)) 
            seekViewport(vp)
        else pushViewport(vp)
        grid.draw(gtable)
        upViewport()
    }
    if (isTRUE(getOption("BrailleR.VI")) && rlang::is_installed("BrailleR")) {
        print(asNamespace("BrailleR")$VI(x))
    }
    invisible(x)
}
<bytecode: 0x55a50675de40>
<environment: namespace:ggplot2>