How can I draw a boxplot without boxes in R?

Question

Using R, I'd like to plot two boxplots without the boxes—just the points.

Creating clean boxplots in R is trivial:

    business <- runif(50, min = 65, max = 100)
    law <- runif(50, min = 60, max = 95)

    boxplot(business, law, horizontal=TRUE, names=
      c("Business", "Law"), col=c('green', 'red'), 
      main="Salary example (boxplot)")

However, the only way I've found to plot just the points in the two random distributions seems needlessly complicated: I overlay two scatterplots with each variable plotted against either 1 or 2, to make a flat line:

    plot(business, rep(1, length(business)), 
          xlim=range(business, law), ylim=c(0, 3), pch=20, 
          col='green', main="Salary example (dots)")
    points(law, rep(2, length(law)), col='red', pch=20)

While this works, it will require a ton more tweaking to get the axes, tickmarks, and labels to match what R does with boxplot(). It seems that there has to be a simpler, more R-like way to do this. What's the best way to draw a boxplot without the box and whiskers—just the individual points?

Answer 1

The stripchart function in the graphics library seems to be what you want if you want to plot the data 1 dimensionally for each group. It produces a somewhat basic plot but you can customize it

    business <- runif(50, min = 65, max = 100)
    law <- runif(50, min = 60, max = 95)
    df <- data.frame(group = rep(c("Business", "Law"), 
            each = 50), value = c(business, law), 
            stringsAsFactors = FALSE)
    
    stripchart(value ~ group, data = df, 
       main = "Salary Example (dots)",
       pch = 16,
       col = c("red", "green"))

Answer 2

One interesting application of R's stripchart() is that you can use jittering or stacking when there is some overlap in data points (see method=). With lattice, the corresponding function is stripplot(), but it lacks the above method argument to separate coincident points (but see below fo one way to achieve stacking).

An alternative way of doing what you want is to use Cleveland's dotchart. Here are some variations around this idea using lattice:

    my.df <- data.frame(x=sample(rnorm(100), 100, replace=TRUE), 
                        g=factor(sample(letters[1:2], 100, 
                        replace=TRUE)))
    library(lattice)
    dotplot(x ~ g, data=my.df)               # g on the x-axis
    dotplot(g ~ x, data=my.df, aspect="xy")  # g on the y-axis
    ## add some vertical jittering (use `factor=` to change 
    ## its amount in both case)
    dotplot(g ~ x, data=my.df, jitter.y=TRUE)  
    stripplot(g ~ x, data=my.df, jitter.data=TRUE)  
    ## use stacking (require the `HH` package)
    stripplot(g ~ x, data=my.df, panel=HH::panel.dotplot.tb, 
                   factor=.2)
    ## using a custom sunflowers panel, available through
    ## http://r.789695.n4.nabble.com/ Grid- graphics- 
    ## issues- tp797307p797307.html
    stripplot(as.numeric(g) ~ x, data=my.df, 
              panel=panel.sunflowerplot, 
              col="black", seg.col="black", seg.lwd=1, size=.08)
    ## with overlapping data, it is also possible 
    ## to use transparency
    dotplot(g ~ x, data=my.df, aspect=1.5, alpha=.5, pch=19)

Some previews of the above commands:

Answer 3

I got a little curious of how the violinplot works when I saw this question. This also led me to the beanplot that might be on the same theme.

The base data creation for all three plots:

business <- runif(50, min = 65, max = 100)
law <- runif(50, min = 60, max = 95)

The violin plot

library(vioplot)
vioplot(business, law, names=c("Business", "Law"), 
        horizontal=T, col=c("lightblue"), rectCol=c('gold'))

Gives below, different colors aren't possible without a tweak:

For getting different colors I found this slightly more advanced solution from Ben Bolker

plot(1,1,ylim=c(0,2.5),xlim=range(c(business, law)),type="n",
     xlab="",ylab="",axes=FALSE)
## bottom axis, with user-specified labels
axis(side=2,at=1:2,labels=c("Business", "Law"))
axis(side=1)
vioplot(business,at=1,col="blue",add=TRUE, horizontal=T)
vioplot(law,at=2,col="gold",add=TRUE, horizontal=T)

And it looks like this:

The beanplot

In my search I also stumbled across the beanplot from Peter Kampstra that seems interesting:

library(beanplot)
beanplot(business, law, horizontal=T, 
         names=c("Business", "Law"), 
         col=c("blue", "gold"))

Gives this: