Layers are a defining feature of the ggplot2 system and its ports (e.g. ggsql, plotnine, gribouille). Wickham 2010 used the term ‘layered’ to differentiate between his implementation and the grammar of graphics conception Wilkinson’s put forth in his seminal work ’The Grammar of Graphics.”
Among the layers that ggplot2 makes available for users, only a
handful of layers are described as making graphical primitives, and
don’t involve pulling in lower level definitions:
geom_blank(), geom_ribbon(),
geom_path() and geom_rect(),
geom_point(),
library(tidyverse)