Guide custom order

[teunbrand]

This PR aims to fix #6195.

Briefly, it applies the order argument, which is encoded into the names of guides after the merge step.

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

df <- tibble::tribble(
  ~x, ~y, ~color, ~shape, 
  1, 2, "grey", 1,
  2, 2, "red", 2
) |>
  dplyr::mutate(shape = as.factor(shape))
circle <- grid::circleGrob()
custom <- guide_custom(circle, title = "My circle", order = 1,
                       width = unit(2, "cm"), height = unit(2, "cm"))
color_legend <- guide_legend(order = 3)
shape_legend <- guide_legend(order = 2)

ggplot(df, aes(x, y, color = color, shape = shape)) +
  geom_point() + 
  guides(color = color_legend, 
         shape = shape_legend,
         custom = custom)

^{Created on 2024-11-25 with reprex v2.1.1}

[thomasp85]

LGTM