How to find centres of stacked bar elements in ggplot?

Question

I posted here about what I'm trying to achieve Sankey bar graphs in R

I've decided to try and implement this in ggplot, but my current issue is this: Given the code below, how would you go about computing the curves df directly from the main df? Is it possible to extract the centres of the stacked geom_bar elements from the plot itself?

df <- data_frame(a=factor(c(1,2,1,2)), b=factor(c(3,3,4,4)), y = c(20,22,24,18))
p <- df %>% 
  ggplot(aes(x=a, y=y, group=b, fill=b, width=.3)) + 
  geom_bar(stat="identity", alpha=.9)
p

curves = data_frame(x=c(1,1), xend=c(2,2), 
                    y=c(10, 35), yend=c(30,30), 
                    weight=c(40,40),
                    width=c(10,40),
                    b=NA) 

p + 
  geom_curve(data=curves, 
             aes(x=x, y=y, xend=xend, yend=yend, size=weight), 
             curvature=.2, alpha=.4)

[related](http://stackoverflow.com/questions/9968433/sankey-diagrams-in-r) — Axeman, Jan 16 '17 at 15:21
Perhaps, you may find [this answer](http://stackoverflow.com/a/41655418/3817004) to the Q "Draw lines between different elements in a stacked bar plot" useful. It pulls data from the plot object. — Uwe, Jan 16 '17 at 17:04

score 0 · Accepted Answer · answered Jan 16 '17 at 15:33

As a quick example how to get the center of the bars:

library(tidyverse)
curves2 <- df %>% 
  group_by(a) %>% 
  mutate(y = cumsum(y) - 0.5 * y) %>% 
  spread(a, y) %>% 
  rename(y = `1`, yend = `2`) %>% 
  mutate(x = 1, xend = 2)

p + geom_curve(data=curves2, 
             aes(x=x, y=y, xend=xend, yend=yend, fill=NA,group=b), 
             curvature=.2, alpha=.4, size=12)

How to find centres of stacked bar elements in ggplot?

1 Answers1