Putting dotplots for two distinct groups on either side of the x-axis

aligning_two_sided_dotplots.R

library(ggplot2)
library(dplyr)
library(magrittr)

# generate fake data
total_N_obs <- 1000
example_data <- 
  data.frame(
    year = sample(2000:2020, replace = TRUE, size = total_N_obs),
    type = sample(c('a', 'b'), replace = TRUE, size = total_N_obs)
  )

# skew the data towards 2020
example_data %<>% rowwise() %>% mutate(
  year = year + sample(0:(2020-year), size = 1))

# make plot
ggplot() + 
  geom_dotplot(
    data = example_data %>% filter(type == 'a'),
    mapping = aes(x = year, fill = type),
    stackdir = 'up',
    method = 'histodot'
  ) + 
  geom_dotplot(
    data = example_data %>% filter(type == 'b'),
    mapping = aes(x = year, fill = type),
    stackdir = 'down',
    method = 'histodot'
  ) + 
  ggtitle("Example plot with dotplots on either side of the x-axis")