This is a demo to introduce how to add grid element into ggplot2 visualization plot.

library(ggplot2)
library(grid)
ggplot(mtcars,aes(x=mpg,y=wt))+geom_point()
current.vpTree()
downViewport("panel.7-5-7-5")
pushViewport(dataViewport(mtcars$mpg,mtcars$wt))
grid.text(x=12.5,y=4.5,default.units = "native",label="Jonie",just=c("left","top"))
grid.points(x=12.5,y=4.5,gp=gpar(col="red"),default.units = "native")