Endpoint stat for ggplot

endpoint.R

StatEndpoint <- ggproto("StatEndpoint", Stat,
  compute_group = function(data, scales) {
    # sort by x so indexing is meaningful
    data = arrange(data, x)
    # grab only the first and last row
    data[c(1,nrow(data)),]
  },
  
  required_aes = c("x", "y")
)


stat_endpoint <- function(mapping = NULL, data = NULL, geom = "point",
                       position = "identity", na.rm = FALSE, show.legend = NA, 
                       inherit.aes = TRUE, ...) {
  layer(
    stat = StatEndpoint, data = data, mapping = mapping, geom = geom, 
    position = position, show.legend = show.legend, inherit.aes = inherit.aes,
    params = list(na.rm = na.rm, ...)
  )
}


geom_line_with_endpoint <- function(..., line.params = list(), endpoint.params = list()) {
  params <- list(...)
  line.params <- modifyList(params, line.params)
  endpoint.params  <- modifyList(params, endpoint.params)
  
  line <- do.call("geom_line", modifyList(
    list(),
    line.params)
  )
  endpoints <- do.call("stat_endpoint", modifyList(
    list(size = 5), # our defaults
    endpoint.params)
  )

  list(line, endpoints)
}


# Create data
tibble(
  g = rep(c('A','B'), each = 50),
  z = rnorm(length(g))
  ) |>
  group_by(g) |>
  mutate(
    y = cumsum(z),
    x = row_number() + g %in% 'B' # offset a bit
    ) %>%
  ungroup() -> tbl

tbl |>
  ggplot(aes(x=x,y=y,group=g,color=g)) + 
  geom_line_with_endpoint()