geom_area does not align with geom_line with duplicate x coords

[banbh]

When x coordinates are duplicated geom_area does not align with geom_line. For example, in the following example, some of the x=2 coordinates are shifted back slightly (but visibly).

library(ggplot2) # ggplot2_3.4.1

data.frame(x = c(1,2,2,3), y = 4:1) |> 
  tidyr::crossing(id = c(0, 1e2)) |>
  ggplot(aes(x + id, y)) +
  facet_wrap(vars(id), scales = 'free_x') +
  geom_area() + # not aligned with lines from next geom_line
  geom_line()

[teunbrand]

Thanks for the report, I can reproduce the issue. Here is the plot rendered:

library(ggplot2)

data.frame(x = c(1,2,2,3), y = 4:1) |> 
  tidyr::crossing(id = c(0, 1e2)) |>
  ggplot(aes(x + id, y)) +
  facet_wrap(vars(id), scales = 'free_x') +
  geom_area() + # not aligned with lines from next geom_line
  geom_line()

^{Created on 2023-03-13 with reprex v2.0.2}

Ultimately, I think this turns out less than ideal due to the fudge factor of 0.001 used in this line:

ggplot2/R/stat-align.R

Line 49 in 04a5ef2

adjust <- diff(range(unique_loc, na.rm = TRUE)) * 0.001

While each panel has a range of 2 on the x-axis, this fudge factor becomes 0.001 * 103 = 0.103 units, which is visible. I think it might be best to do the alignment calculations per panel instead of for the data as a whole. The stacking, that stat = "align" makes easier, applies on a panel-level rather than the whole data level as well.