Adding point to legend with ggplot2 and geom_sf - r

Question

I am creating a map and then adding some cities on top of it, and I want to have multiple legend items.
So far I have this code:
library(tidyverse)
library(raster)
library(sf)
library(maptools)
#a location to add to the map
city <- tibble(y = c(47.7128),
x = c(74.0060))
city <- st_as_sf(city, coords = c("x", "y"), crs = "+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs +towgs84=0,0,0")
#world map to plot, along with a raster of distance from a point
data(wrld_simpl)
wrld_simpl_sf <- sf::st_as_sf(wrld_simpl)
r <- raster(wrld_simpl, res = 1)
wrld_r <- rasterize(wrld_simpl, r)
#
pt1 <- matrix(c(100,0), ncol = 2)
dist1 <- distanceFromPoints(r, pt1)
values(dist1)[values(dist1) > 5e6] <- NA
plot(dist1)
gplot_data <- function(x, maxpixels = 50000) {
x <- raster::sampleRegular(x, maxpixels, asRaster = TRUE)
coords <- raster::xyFromCell(x, seq_len(raster::ncell(x)))
## Extract values
dat <- utils::stack(as.data.frame(raster::getValues(x)))
names(dat) <- c('value', 'variable')
dat <- dplyr::as.tbl(data.frame(coords, dat))
if (!is.null(levels(x))) {
dat <- dplyr::left_join(dat, levels(x)[[1]],
by = c("value" = "ID"))
}
dat
}
gplot_dist1 <- gplot_data(dist1)
gplot_wrld_r <- gplot_data(wrld_r)
#plot data
ggplot() +
geom_sf(data = wrld_simpl_sf, fill = "grey20",
colour = "white", size = 0.2) +
geom_tile(data = gplot_dist1,
aes(x = x, y = y, fill = value)) +
scale_fill_gradient("Distance",
low = 'yellow', high = 'blue',
na.value = NA) +
geom_sf(data = city, fill = "red", color = "red", size = 3, shape = 21)
which returns:
This is all fine but now I just want to add the red point from geom_sf(data = city, fill = "red", color = "red", size = 3, shape = 21) to the legend with the caption "Cities".

Answer 1

You could use the scale_color_manual function. The way I understand (I found out about this today), it allows you to map colors to "levels" that then appear in the legend.
Changing your code to have the following does the trick.
geom_sf(data = city, fill = "red", aes(color = "Cities"), size = 3, shape = 21) +
scale_color_manual(values = c("Cities" = "red"))
This is the result