Groupwise percentage in ggparty node plots

Question

I'm working with ggparty and trying to customize the bar charts at each terminal node of a partykit plot (with geom_node_plot). I want the lengths of some bars (and their labels) to show a proportion of the total predicted cases -- where the denominator in that percentage is from cases at that node only, not out of the entire data set.

(This is a common enough question within ggplot2 alone, but the difficulty here is specific to working inside of ggparty.)

require(ggplot2)
require(partykit)
require(ggparty)
    
data("WeatherPlay", package = "partykit")
wptree <- ctree(play ~ outlook, data=WeatherPlay, control = ctree_control(minsplit=1, minbucket=1, mincriterion = .01))
    
ggparty(wptree) +
      geom_edge() +
      geom_edge_label() +
      geom_node_splitvar() +
      geom_node_plot(gglist = list(
        aes(
          y = play,
          x = after_stat(count / sum(count))
        ),
        geom_bar()
      ))

The vignette for ggparty states that each node plot is "separate", suggesting that the data are segregated, but this doesn't seem to apply to count , for as you can see, the percentages are out of all data in the dataset.

I know that a slightly alternate method would be to use prop instead of count / sum(count) and this allows the "by" parameter to specify the grouping. That seems ideal -- but what is the grouping variable available here, that would segregate the nodes? The party nodeid does not seem to be in the environments for each geom_bar call.

Any custom groupwise calculation, eg with lapply, also would require a variable to use, specifying the terminal node, or otherwise distinguishing between plots -- so that seems to be the key issue.

Thanks for any guidance.

Answer 1

As far as I get it the bars for the nodes are added as a facetted plot. To get the percentages by panel you could basically use the same approach as for a standard ggplot, ie as you already guessed you have to take account of the grouping which for the panels you could get from PANEL :

library(ggparty)

data("WeatherPlay", package = "partykit")
wptree <- ctree(play ~ outlook, data=WeatherPlay, control = ctree_control(minsplit=1, minbucket=1, mincriterion = .01))

panel_prop <- function(count, panel) {
  count / tapply(count, panel, sum)[as.character(panel)]
}

ggparty(wptree) +
  geom_edge() +
  geom_edge_label() +
  geom_node_splitvar() +
  geom_node_plot(gglist = list(
    aes(
      y = play,
      x = after_stat(panel_prop(count, PANEL))
    ),
    geom_bar()
  ))