Combine plots of different classes in R

Question

I am working with plots of different classes in r, namely tmap (main plots) and ggplot (represent minor information).

library(raster)
library(tmap)
library(RStoolbox)
library(gridExtra)
library(ggplot2)

data("World")
r1 <- raster(ncol=2, nrow=2)
values(r1) <- c(1.5, -0.5, 0.5, -1.5)

r2 <- raster(ncol=2, nrow=2)
values(r2) <- c(1, 1, 0.5, -1.5)

# Then I create two tmap objects and two ggplot objects
tm1 <- tm_shape(World) + tm_polygons()  
tm2 <- tm_shape(World) + tm_polygons()

gg1 <- ggR(r1,  coord_equal = F, geom_raster = 1) + theme(legend.position = "none", axis.title = element_blank(), axis.text = element_text(size = 3.5))
gg2 <- ggR(r2,  coord_equal = F, geom_raster = 1) + theme(legend.position = "none", axis.title = element_blank(), axis.text = element_text(size = 3.5))

# Save each separately
# 1 tmap
current.mode <- tmap_mode("plot")
tmap_plot <- tmap_arrange(tm1, tm2, ncol = 1, nrow = 2)
tmap_save(tmap_plot, "tmap_plot.png", height = 8, width = 6)

# 2 ggplot
g <- arrangeGrob(gg1, gg2)
ggsave(file = "ggplot.png", g, height = 1.75, width = 1.75)

Currently, I save each class separately and combine them by using manual editing tools. I am looking for a solution, how to automate that process already in r. I am also open for suggestions, how to do it in other programming languages, just that it automates the process. Switching to different classes is not an option in my case.

Answer 1

You can draw the ggplots directly over the tmaps using a grid::viewport.

First, load grid and set the desired viewport location and dimensions:

library(grid)

subplot_vp <- viewport(x = 0.15, 
                       y = 0.35, 
                       width = unit(0.25, "npc"), 
                       height = unit(0.25, "npc"),
                       name = "inset")

Now you can do

tm1
grid.draw(grobTree(ggplotGrob(gg1), vp = subplot_vp))

and

tm2
grid.draw(grobTree(ggplotGrob(gg2), vp = subplot_vp))