Question

We can draw box plot as below:

qplot(factor(cyl), mpg, data = mtcars, geom = "boxplot")

and point as:

qplot(factor(cyl), mpg, data = mtcars, geom = "point") 

How would you combine both – but just to show a few specific points(say when wt is less than 2) on top of the box?

Answer 1

Use + geom_point(...) on your qplot (just add a + geom_point() to get all the points plotted).

To plot selectively just select those points that you want to plot:

n <- nrow(mtcars)
# plot every second point
idx <- seq(1,n,by=2)

qplot( factor(cyl), mpg, data=mtcars, geom="boxplot" ) +
     geom_point( aes(x=factor(cyl)[idx],y=mpg[idx]) )    # <-- see [idx] ?

If you know the points before-hand, you can feed them in directly e.g.:

qplot( factor(cyl), mpg, data=mtcars, geom="boxplot" ) +
     geom_point( aes(x=factor(c(4,6,8)),y=c(15,20,25)) ) # plot (4,15),(6,20),...