Plot multiple series of data into a single bagplot with R

Question

Let's condsider the bagplot example as included in the aplpack library in R. A bagplot is a bivariate generalisation of a boxplot and therefore gives insight in the distribution of data points in both axes.

Example of a bagplot:

Code for the example:

  # example of Rousseeuw et al., see R-package rpart
  cardata <- structure(as.integer( c(2560,2345,1845,2260,2440,
   2285, 2275, 2350, 2295, 1900, 2390, 2075, 2330, 3320, 2885,
   3310, 2695, 2170, 2710, 2775, 2840, 2485, 2670, 2640, 2655,
   3065, 2750, 2920, 2780, 2745, 3110, 2920, 2645, 2575, 2935,
   2920, 2985, 3265, 2880, 2975, 3450, 3145, 3190, 3610, 2885,
   3480, 3200, 2765, 3220, 3480, 3325, 3855, 3850, 3195, 3735,
   3665, 3735, 3415, 3185, 3690, 97, 114, 81, 91, 113, 97, 97,
   98, 109, 73, 97, 89, 109, 305, 153, 302, 133, 97, 125, 146,
   107, 109, 121, 151, 133, 181, 141, 132, 133, 122, 181, 146,
   151, 116, 135, 122, 141, 163, 151, 153, 202, 180, 182, 232,
   143, 180, 180, 151, 189, 180, 231, 305, 302, 151, 202, 182,
   181, 143, 146, 146)), .Dim = as.integer(c(60, 2)), 
   .Dimnames = list(NULL, c("Weight", "Disp.")))
  bagplot(cardata,factor=3,show.baghull=TRUE,
    show.loophull=TRUE,precision=1,dkmethod=2)
  title("car data Chambers/Hastie 1992")
  # points of y=x*x
  bagplot(x=1:30,y=(1:30)^2,verbose=FALSE,dkmethod=2)

The bagplot of aplpack seems to only support plotting a "bag" for a single data series. Even more interesting would be to plot two (or three) data series within a single bagplot, where visually comparing the "bags" of the data series gives insight in the differences in the data distributions of the data series. Does anyone know if (and if so, how) this can be done in R?

Answer 1

If we modify some of the aplpack::bagplot code we can make a new geom for ggplot2. Then we can compare groups within a dataset in the usual ggplot2 ways. Here's one example:

library(ggplot2)
ggplot(iris, aes(Sepal.Length, Sepal.Width, 
                 colour = Species, fill = Species)) +
       geom_bag() +
       theme_minimal()

and we can show the points with the bagplot:

ggplot(iris, aes(Sepal.Length, Sepal.Width, 
                     colour = Species, fill = Species)) +
           geom_bag() +
           geom_point() + 
           theme_minimal()

Here's the code for the geom_bag and modified aplpack::bagplot function: https://gist.github.com/benmarwick/00772ccea2dd0b0f1745