How to represent different borders in tmap legend-CodePudding

my map looks similar to this one:

# import shapefile
shape_data <- system.file("shape/nc.shp", package="sf")
shape_data <- st_read(shape_data) 
sample_data <- filter(shape_data, CNTY_ID >2100)
shape_area <- st_union(shape_data) 
#map data
tm_shape(shape_data)   tm_borders(alpha = 0.4,lwd = 0.1, col = "blue")  
tm_fill(col = "BIR79", style = "equal")  
tm_shape(sample_data)    tm_borders(col = "red", lwd = 3, lty = 4)  
tm_shape(shape_area)   tm_borders(col = "black", lwd = 2)

the polygons in the map are distinguished by three types of boundaries, with different colours, size and, in one case, by dashes. Now, I would like to find a way to represent these 3 boundaries in a legend, either in addition to the data legend or even separate. So I would like to have the representation of the type of boundaries (red dashed, black and blue with their different size, and next to them add a text explaining what they represent, i.e. districts, boundaries of the study area and so on.

Do you have any idea how to do this with tmap? Thanks

CodePudding user response：

Here's a solution using the viewport function from the grid package:

library(tmap)
library(sf)
library(tidyverse)
library(grid)

shape_data <- system.file("shape/nc.shp", package="sf")
shape_data <- st_read(shape_data) 
sample_data <- filter(shape_data, CNTY_ID >2100)
shape_area <- st_union(shape_data)

tm1 <- tm_shape(shape_data)   
  tm_fill(col = "BIR79", style = "equal")  
  tm_shape(shape_data)   tm_borders(alpha = 0.4,lwd = 1.5, col = "blue")  
  tm_shape(sample_data)   tm_borders(col = "red", lwd = 3, lty = 4)  
  tm_shape(shape_area)   tm_borders(col = "black", lwd = 2) 

# add second legend
tm_leg = tm_shape(sample_data)   tm_fill(alpha = 0)   # dummy layer
  tm_add_legend("line", 
                col = c("blue", "red", "black"), 
                lwd = c(1.5, 3, 2), 
                lty = c(1, 4, 1),
                labels = c("var1", "var2", "var3"),
                title = "type")  
  tm_layout(legend.position = c(.15 ,-.2), 
            bg.color = NA, 
            frame = F, 
            legend.width = 10, 
            legend.height = 10)

tmap_save(tm1, insets_tm = tm_leg, insets_vp = viewport(), "test_map.png")