Add circular reference line to polar bar chart in ggplot2

Question

I'm attempting to add a circle using geom_polygon() to add a reference line to radial bar chart. (I know this is not the ideal way to present this data, but it's not entirely up to me). Following this issue , I can create a nice circle:

library(tidyverse)

lines <- tibble(
  rating = rep(1:7, times = 3),
  average = rep(1:3, each = 7)
)

ggplot(lines, aes(x = rating, y = average)) +
  geom_polygon(aes(group = average), fill = NA, color = "black") + 
  scale_x_continuous(breaks = 1:7) +
  scale_y_continuous(limits = c(0, 3), breaks = 0:3) +
  coord_polar()

This is great. The problem is that 1 and 7 share the top spot. So when I add data for the bars, the circle flattens out.

set.seed(123)
bars <- tibble(rating = 1:7,
               average = runif(7, min = 0, max = 3))

ggplot(lines, aes(x = rating, y = average)) +
  geom_col(data = bars, aes(fill = factor(rating)), show.legend = FALSE) +
  geom_polygon(aes(group = average), fill = NA, color = "black") + 
  scale_x_continuous(breaks = 1:7) +
  scale_y_continuous(limits = c(0, 3), breaks = 0:3) +
  coord_polar()

Is there any way keep the polygon a circle in this situation? Or to interpolate between the 7 and 1 to keep the original curve?

^{Created on 2021-07-26 by the reprex package (v2.0.0)}

Session info

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Answer 1

One option would be to make use of geom_hline instead of geom_polygon :

library(tidyverse)

set.seed(123)
lines <- tibble(
  rating = rep(1:7, times = 3),
  average = rep(1:3, each = 7)
)

bars <- tibble(rating = 1:7,
               average = runif(7, min = 0, max = 3))

ggplot(lines, aes(x = rating, y = average)) +
  geom_col(data = bars, aes(fill = factor(rating)), show.legend = FALSE) +
  geom_hline(aes(yintercept = average)) +
  scale_x_continuous(breaks = 1:7) +
  scale_y_continuous(limits = c(0, 3), breaks = 0:3) +
  coord_polar()