Can you stack two different ggmap maps?

Question

I have a project in which I plot some data over maps obtained with ggmap. All works very well, except that none of the map varieties you can obtain completely fulfills my need. Ideally I would like to have the two stamen map types "watercolor" and "terrain-labels" fused together before I uses them as the background for plotting with ggplot. I can obtain the 2 geographically mathcing maps with:

library("ggmap")
library("ggplot2")
lon<-c(-71.50,-71.60)
lat<-c(42.40,42.50)
coords<-data.frame(lon,lat)
newmap1<-get_stamenmap(bbox = c(left = -71.63, bottom = 42.36, 
right = -71.10, top = 42.55), maptype="watercolor", zoom=13)

newmap2<-get_stamenmap(bbox = c(left = -71.63, bottom = 42.36, 
right = -71.10, top = 42.55), maptype="terrain-labels", zoom=13)

bbleft, bbright, bbbottom, bbtop indicate any set of meaningful coordinates. Now I would like to stack newmap2 over newmap1 (with alpha 0.5, for example) and then use that to plot my data on as I would usually do with a single map:

ggmap(newmap1) + geom_point(aes(x = lon, y = lat), data = coords, colour = "#f409d8", size = 1, alpha =0.8)

Thank you.

I have tried luke's code below, but this is the image I get: enter image description here

Luke: I now updated all the packages and your code (transferred to my data) works (Thank you vey much for that!!!):

library("ggmap")
library("ggplot2")
lon<-c(-71.50,-71.60)
lat<-c(42.40,42.50)
coords<-data.frame(lon,lat)
newmap1<-get_stamenmap(bbox = c(left = -71.63, bottom = 42.36, right = -71.10, top = 42.55), maptype="watercolor", zoom=12)

newmap2<-get_stamenmap(bbox = c(left = -71.63, bottom = 42.36, right = -71.10, top = 42.55), maptype="toner-lite", zoom=12)
newmap2_ <- adjustcolor(newmap2, alpha.f = .5)
attributes(newmap2_) <- attributes(newmap2)
map <- expand.grid(lon = as.numeric(attr(newmap1, "bb")[, c("ll.lon", "ur.lon")]), lat = as.numeric(attr(newmap1, "bb")[, c("ll.lat", "ur.lat")]))
xmin <- attr(newmap1, "bb")$ll.lon
xmax <- attr(newmap1, "bb")$ur.lon
ymin <- attr(newmap1, "bb")$ll.lat
ymax <- attr(newmap1, "bb")$ur.lat
ggplot () + 
geom_blank(aes(x = lon, y = lat), data = map) + 
annotation_raster(newmap1, xmin, xmax, ymin, ymax) + 
annotation_raster(newmap2_, xmin, xmax, ymin, ymax)

Only problem, the proportions of the map are wrong, with the latitude overstretched, resulting in an abnormal looking map: Map showing merging of the two maps Is there any workaround?

Answer 1

Here's a reproducible example that simulates alpha = .5:

library(ggmap)
bbox <- c(left = -97.132, bottom = 31.536, right = -97.105, top = 31.560)
m1 <- get_stamenmap(bbox, maptype = "watercolor", zoom = 13)
m2 <- get_stamenmap(bbox, maptype = "terrain-labels", zoom = 13)
m2_ <- adjustcolor(m2, alpha.f = .5)
attributes(m2_) <- attributes(m2)
fourCorners <- expand.grid(lon = as.numeric(attr(m1, "bb")[, c("ll.lon", "ur.lon")]), lat = as.numeric(attr(m1, "bb")[, c("ll.lat", "ur.lat")]))
xmin <- attr(m1, "bb")$ll.lon
xmax <- attr(m1, "bb")$ur.lon
ymin <- attr(m1, "bb")$ll.lat
ymax <- attr(m1, "bb")$ur.lat
ggplot() + 
  geom_blank(aes(x = lon, y = lat), data = fourCorners) + 
  annotation_raster(m1, xmin, xmax, ymin, ymax) + 
  annotation_raster(m2_, xmin, xmax, ymin, ymax)