library(tmap)
library(terra)
library(sf)

r <- rast(system.file("ex/elev.tif", package="terra"))

states <- st_read(system.file("ex/lux.shp", package="terra"))

# make some bbox magic
bbox_new <- st_bbox(r) # current bounding box

xrange <- bbox_new$xmax - bbox_new$xmin # range of x values
yrange <- bbox_new$ymax - bbox_new$ymin # range of y values

bbox_new[1] <- bbox_new[1] - (0.05 * xrange) # xmin - left
bbox_new[3] <- bbox_new[3] + (0.05 * xrange) # xmax - right
bbox_new[2] <- bbox_new[2] - (0.05 * yrange) # ymin - bottom
bbox_new[4] <- bbox_new[4] + (0.05 * yrange) # ymax - top

bbox_new <- bbox_new %>%  # take the bounding box ...
  st_as_sfc() # ... and make it a sf polygon


tm_shape(r, bbox = bbox_new) +
  tm_raster(col.scale = tm_scale_continuous(
    values = "viridis"),  # color palette; 
    col.legend = tm_legend(title = "Legend", frame.lwd = 0,
                           position = c("LEFT", "BOTTOM"))) +
  tm_shape(states) +
  tm_borders() +
  tm_layout(panel.labels = expression(bold("Elevation (No. m"^-2*")"))) #Fake unit just to show