New position adjustment: position_connect()

[teunbrand]

This PR aims to fix #6228.

It introduces a new position adjustment that can be used to connect points via steps or other shapes.

It is probably best introduced with some examples:

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2

# Mirroring `geom_step()`
ggplot(head(economics, 20), aes(date, unemploy)) +
  geom_line(position = "connect")

# Making a histogram without bars
ggplot(faithful, aes(waiting)) +
  geom_area(
    stat = "bin", bins = 20, pad = TRUE,
    position = position_connect("mid")
  )

# Using custom connections with a matrix.
# Note that point A at (0, 0) is not included, but point B at (1, 1) is.
zigzag <- cbind(c(0.4, 0.6, 1), c(0.75, 0.25, 1))
x <- seq(0, 1, length.out = 20)[-1]
smooth <- cbind(x, scales::rescale(1 / (1 + exp(-(x * 10 - 5)))))

ggplot(head(economics, 10), aes(date, unemploy)) +
  geom_line(position = position_connect(zigzag), aes(colour = "zigzag")) +
  geom_line(position = position_connect(smooth), aes(colour = "smooth")) +
  geom_point()

^{Created on 2024-12-11 with reprex v2.1.1}

[teunbrand]

Per discussion in the linked issue, here is an example of a KM curve next to ggsurvplot (who I trust have implemented this correctly for KM curves).

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2
library(survival)
model <- survfit(Surv(time, status) ~ x, data = aml)

df <- broom::tidy(model)

ggplot(df, aes(time, estimate, ymin = conf.low, ymax = conf.high)) +
  geom_step(aes(colour = strata)) +
  geom_ribbon(
    aes(fill = strata), alpha = 0.2,
    position = "connect"
  )

ggsurvfit::ggsurvfit(model) +
  ggsurvfit::add_confidence_interval() +
  theme_gray()
#> Warning: Ignoring unknown labels:
#> • `linetype = "NULL"`

^{Created on 2024-12-11 with reprex v2.1.1}

I suspect ggsurvfit does some extra steps to ensure series start at 'time = 0' and 'probability = 1', but the CI ribbons look not too bad.