# A standard plot
p <- ggplot(mpg, aes(interaction(drv, year), displ)) +
geom_point()
key <- key_range_manual(c(2, 4), c(5, 6), c("A", "B"))
# Adding as secondary guides
p + guides(
x.sec = primitive_box(),
y.sec = primitive_box(key = key)
)
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