xmod and ymod parameters of tm_text do not preserve along.lines

[agila5]

Hi! I was creating some maps with lines and text and I found this weird behaviour. The following should be a reprex of the problem. First I create some data with lines and text

# packages
library(sf)
#> Linking to GEOS 3.6.1, GDAL 2.2.3, PROJ 4.9.3
library(tmap)

# create some fake data
my_sfc <- st_sfc(
  st_linestring(matrix(c(0, -2, 0, -1, 0, 0), ncol = 2, byrow = TRUE)), 
  st_linestring(matrix(c(0, 2, 0, 1, 0, 0), ncol = 2, byrow = TRUE)), 
  st_linestring(matrix(c(-2, 0, -1, 0, 0, 0), ncol = 2, byrow = TRUE)), 
  st_linestring(matrix(c(2, 0, 1, 0, 0, 0), ncol = 2, byrow = TRUE))
)

my_sf <- st_sf(
  data.frame(my_text = c("1", "2", "3", "4")), 
  geometry = my_sfc
)

Then I create the map and everything is fine.

tm_shape(my_sf) + 
  tm_lines() + 
  tm_text("my_text", size = 2, along.lines = TRUE)
#> Warning: Currect projection of shape my_sf unknown. Long-lat (WGS84) is assumed.

The problems occur when I try to modify xmod and ymod to move the labels since, in this case, they do not preserve the along.lines = TRUE option.

tm_shape(my_sf) + 
  tm_lines() + 
  tm_text(
    "my_text", 
    size = 2, 
    along.lines = TRUE, 
    xmod = c(1, 1, 0, 0), 
    ymod = c(0, 0, 1, 1)
  )
#> Warning: Currect projection of shape my_sf unknown. Long-lat (WGS84) is assumed.

^{Created on 2019-11-26 by the reprex package (v0.3.0)}

Is this a bug? If not, is it possible to solve the problem? Thanks

[mtennekes]

Not really a bug, but a result of the current implementation. For text post-processing, namely auto.placement, remove.overlap, along.lines and overwrite.lines, grobs are used for calculation (e.g. of the rotation angles): https://github.com/mtennekes/tmap/blob/master/R/plot_map.R#L94-L247. I am not happy with this implementation.

It requires some effort to re-implement.

[Nowosad]

This problem is fixed in the upcoming tmap v4:

library(sf)
library(tmap)

# create some fake data
my_sfc <- st_sfc(
  st_linestring(matrix(c(0, -2, 0, -1, 0, 0), ncol = 2, byrow = TRUE)), 
  st_linestring(matrix(c(0, 2, 0, 1, 0, 0), ncol = 2, byrow = TRUE)), 
  st_linestring(matrix(c(-2, 0, -1, 0, 0, 0), ncol = 2, byrow = TRUE)), 
  st_linestring(matrix(c(2, 0, 1, 0, 0, 0), ncol = 2, byrow = TRUE))
)

my_sf <- st_sf(
  data.frame(my_text = c("1", "2", "3", "4")), 
  geometry = my_sfc,
  crs = "EPSG:4326"
)

tm_shape(my_sf) + 
  tm_lines() + 
  tm_text("my_text", size = 2, along.lines = TRUE)

tm_shape(my_sf) +
tm_lines() + 
  tm_text(
    "my_text", 
    size = 2, 
    along.lines = TRUE, 
    xmod = c(1, 1, 0, 0), 
    ymod = c(0, 0, 1, 1)
  )

^{Created on 2023-02-18 with reprex v2.0.2}