Plot two layers (raster and polygon-vector) with ggplot and use the fill-scale* for both layers

Question

I am a little lost. I have a stars object where I would like to use the first attribute and map it to the scale_fill*-function in ggplot.

I also have an polygon-object of class sf that has an attribute that I would also like to map to the scale_fill*-function.

I really tried to compile some data and a reproducible example and I got this far: It does not make any sense at all, but serves for the purpose:

library(stars)
library(tidyverse)
library(raster)
library(osmdata)
library(scico)

# get elevation
alt <- getData('alt', country='CHE')
alt_stars = st_as_stars(alt)

ch_canvas = st_bbox(alt_stars) %>% opq()

# overpass query
ch_moto = ch_canvas %>%
  add_osm_feature(key = "highway",
                  value="motorway") %>% 
  osmdata_sf() 

# get the sf object
df = ch_moto %>% pluck("osm_lines") %>% 
  dplyr::select("maxspeed") %>% 
  filter(!is.na(maxspeed),
         maxspeed != "none") %>% 
  mutate(maxspeed = as.numeric(maxspeed))

# make the polygons
df_buffer = st_buffer(df, 0.1)

ggplot() +
  geom_stars(
    data = alt_stars,
    aes(x=x, y=y, fill = CHE_msk_alt),
    downsample = 2
  ) +
  scale_fill_continuous(low="white", high="red") +
  geom_sf(
    data = df,
    aes(fill=maxspeed)
  ) +
  scale_fill_scico(palette="berlin")

I am then getting this message: Scale for 'fill' is already present. Adding another scale for 'fill', which will replace the existing scale.

I would just like to know how to plot any two layers (in many cases a stars-object and a sf-object) on top of each other and fill both according to the values in a variable. I tried to use the cowplot-package and just define two ggplot objects and plot them above each other, but did not manage to make it work. Maybe someone has a pointer on what to do in these situations:)

Answer 1

Sounds like you are looking for something like the functionality in the ggnewscale package. In your case, you could use its new_scale_fill() function, like this:

library(ggnewscale)

# make the polygons (using a smaller buffer)
df_buffer = st_buffer(df, 0.01)

ggplot() +
  geom_stars(
    data = alt_stars,
    aes(x = x, y = y, fill = CHE_msk_alt),
    downsample = 2
  ) +
  scale_fill_continuous(low = "white", high = "red") +
  new_scale_fill() + ## geoms added after this will use a new scale definition
  geom_sf(
    data = df_buffer, 
    col = NA,
    aes(fill = maxspeed)
  ) +
  scale_fill_scico(palette = "berlin")