How do you position the title and legend in tmap?

Question

I am new to programming and am currently enrolled in an introductory spatial analysis course which uses R. The following code yields the tmaps included below. How can I center the title for each tmap as well as position the legend on the top right without laying over top the maps themselves?

Thank you very much for your assistance.

  ga1 = tm_shape(a2georgia) +
  tm_polygons('PctBlack', style='quantile', breaks=c(4.98,11.75, 22.35,27.64, 32.55, 40.06,   48.18, 79.64),
              n=8, palette=c('lightblue','khaki1', 'red3'), title='Quantiles(8)',
              border.col='grey27', alpha=.9) +
  tm_layout(legend.position = c("right", "top"), title= '% of Population of Black Race', title.position = c('right', 'top'))


ga_cartogram <- cartogram_cont(a2georgia, "PctBlack", itermax=5)

  ga2 = tm_shape(ga_cartogram) + 
  tm_polygons("PctBlack", style='quantile', breaks=c(4.98,11.75, 22.35,27.64, 32.55, 40.06, 48.18, 79.64), 
              n=8, palette=c('lightblue','khaki1', 'red3'), title='Quantiles(8)',
              border.col='grey27', alpha=.9) +
  tm_layout(legend.position = c("right", "top"), title= '% of Population of Black Race',  title.position = c('right', 'top'))


tmap_arrange(ga1,ga2)

Tmaps

The documentation appears to have `main.title.position` with possible value `center`. Have you looked into that? — Roman Luštrik, Mar 27 '20 at 18:49
Just tried that and the legend becomes positioned higher up but still slightly overtop the map. The main title was not affected. — Hubmeister, Mar 27 '20 at 19:35

score 16 · Accepted Answer · answered Mar 28 '20 at 09:57

The problem is that {tmap} draws legend and titles inside the bounding box of your polygon. To make more room you have to extend the bounding box somewhat.

A while back I wrote a blog post on this topic, you might want to have a look at that https://www.jla-data.net/eng/adjusting-bounding-box-of-a-tmap-map/

As your example is not exactly reproducible I will demonstrate the technique on the North Carolina shapefile that is shipped with {sf} and thus widely available.

library(sf)
library(tmap)

# NC counties - a shapefile shipped with the sf package
nc <- st_read(system.file("shape/nc.shp", package ="sf"))


# bad, bad map...
tm_shape(nc) + tm_polygons("NWBIR74", style='quantile', 
                           breaks=c(4.98,11.75, 22.35,27.64, 32.55, 40.06, 48.18, 79.64),
                           n=8, palette=c('lightblue','khaki1', 'red3'), 
                           title='Quantiles(8)',
                           border.col='grey27', alpha=.9) +
  tm_layout(legend.position = c("right", "top"), 
            title= '% of Population of Black Race', 
            title.position = c('right', 'top'))

# make some bbox magic
bbox_new <- st_bbox(nc) # 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] - (0.25 * xrange) # xmin - left
 bbox_new[3] <- bbox_new[3] + (0.25 * xrange) # xmax - right
# bbox_new[2] <- bbox_new[2] - (0.25 * yrange) # ymin - bottom
bbox_new[4] <- bbox_new[4] + (0.2 * yrange) # ymax - top

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

# looks better, does it?
tm_shape(nc, bbox = bbox_new) + tm_polygons("NWBIR74", style='quantile', 
                           breaks=c(4.98,11.75, 22.35,27.64, 32.55, 40.06, 48.18, 79.64),
                           n=8, palette=c('lightblue','khaki1', 'red3'), 
                           title='Quantiles(8)',
                           border.col='grey27', alpha=.9) +
  tm_layout(legend.position = c("right", "top"), 
            title= '% of Population of Black Race', 
            title.position = c('right', 'top'))

Fantastic, thanks for your response. May I ask you to clarify the following? Are you saying that a sf geometry is needed?bbox_new <- bbox_new %>% # take the bounding box ... st_as_sfc() # ... and make it a sf polygon — Hubmeister, Mar 29 '20 at 04:20
The sf::st_as_sfc is necessary because bbox argument of tmap::tm_shape expects a geometry, while sf::st_bbox returns a vector; have a look at {sf} documentation for more detailed explanation https://www.rdocumentation.org/packages/sf/versions/0.8-1/topics/st_bbox — Jindra Lacko, Mar 29 '20 at 10:29

How do you position the title and legend in tmap?

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