R - ggplot2 'dodge' geom_step() to overlap geom_bar()

I think the most efficient way to solve this problem is to define custom geom in the following way:

library(tidyverse)

geom_step_extend <- function(data, extend = 1, nudge = -0.5,
                             ...) {
  # Function for computing the last segment data
  get_step_extend_data <- function(data, extend = 1, nudge = -0.5) {
    data_out <- as.data.frame(data[order(data[[1]]), ])
    n <- nrow(data)
    max_x_y <- data_out[n, 2]
    if (is.numeric(data_out[[1]])) {
      max_x <- data_out[n, 1] + nudge
    } else {
      max_x <- n + nudge
    }

    data.frame(x = max_x,
               y = max_x_y,
               xend = max_x + extend,
               yend = max_x_y)
  }

  # The resulting geom
  list(
    geom_step(position = position_nudge(x = nudge), ...),
    geom_segment(
      data = get_step_extend_data(data, extend = extend, nudge = nudge),
      mapping = aes(x = x, y = y,
                    xend = xend, yend = yend),
      ...
    )
  )
}

set.seed(111)
test <- data_frame(a = 1:10, b = runif(10, 1, 10))
test2 <- data_frame(a = letters[1:10], b = runif(10, 1, 10))

test_plot <- ggplot(test, aes(a, b, group = 1)) + 
  geom_bar(stat = "identity") + 
  geom_step_extend(data = test, colour = "red")

test2_plot <- ggplot(test2, aes(a, b, group = 1)) + 
  geom_bar(stat = "identity") + 
  geom_step_extend(data = test2, colour = "red")

gridExtra::grid.arrange(test_plot, test2_plot, ncol = 2)

Basically this solution consists from three parts:

1. Nudge to the left with position_nudge the step curve by desired value (in this case -0.5);
2. Compute the absent (the one on the right) segment data with function get_step_extend_data. Its behaviour is inspired from ggplot2:::stairstep which is the underlying function of geom_step;
3. Compose geom_step with geom_segment in separate geom with list.

Here's a rather crude solution, but should work in this case.

Create an alternate data frame that expanded each line to extend the x-axis by -0.5 and 0.5:

test2 <- data.frame(a = lapply(1:nrow(test), function(x) c(test[x,"a"]-.5, test[x,"a"], test[x, "a"]+0.5)) %>% unlist, 
                b = lapply(1:nrow(test), function(x) rep(test[x,"b"], 3)) %>% unlist)

Plot the outline with geom_line argument:

ggplot(test, aes(a,b)) + geom_bar(stat="identity", alpha=.7) + geom_line(data=test2, colour="red")

This will look tidier if you set the geom_bar width to 1:

ggplot(test, aes(a,b)) + geom_bar(width=1, stat="identity", alpha=.7) + geom_line(data=test2, colour="red")

Since ggplot2 version 3.3.0 this is option is now supported by geom_step using direction = "mid":

library(tidyverse)

test <- data_frame(a = 1:10, b = runif(10, 1, 10))

test_plot <- ggplot(test, aes(a, b)) + 
  geom_bar(stat="identity") + 
  geom_step(color = 'red', direction = "mid", size = 2)

test_plot