在R / ggplot2中突出显示密度和小提琴图上的个别观察

Question

I am trying to highlight the position of an individual in a density plot and in a violin plot.

For instance, in the plot generated by the code bellow, I would like highlight "Mazda RX4" in the mtcars dataset. This could be by adding a point or bar to the value of that individual observation in each category

#density plot
ggplot(data=mtcars,aes(x=drat))+
  geom_density() + facet_grid(cyl ~ .)

#violin plot
library(ggplot2)
ggplot(data=mtcars,aes(x=factor(cyl),y=drat, fill=factor(am)))+
  geom_violin() 

EDIT: 1) Based on @DatamineR answer bellow I as able to add the desired point (code bellow) to the violin plot.

ggplot(data=mtcars,aes(x=factor(cyl),y=drat, fill=factor(am)))+ 
  geom_violin() + geom_point(data = mtcars[rownames(mtcars) == "Mazda RX4",])

However, the same +geom_point() did not work with the density plot. How can I add a point for that observation for the density plot?

2) Also, is it possible to use another marker, like a line segment or bar (perpendicular to the base line of the distribution) instead of a point?

Answer 1

Try the following for the first part of the question (add to your ggplot-code):

geom_point(data = mtcars[rownames(mtcars) == "Mazda RX4",])

For the second part of the question you can try(add to your ggplot-code):

geom_point(data = mtcars[rownames(mtcars) == "Mazda RX4",], aes(y = 0))