Avoid calling tm_shape() multiple times?

Question

Posting here (rather than Github) as it is not an issue with the tmap package.

Situation:

I am doing simple choropleth maps of varinterest for U.S. counties (N~3000) using tmap (data stored in data object). Each map also has a key column that we will call category. I am rendering a variable of about ~300 counties for a varinterest for a category, but I would also like to fill the counties with missing data as gray.

My current method is as follows:

tm_shape(data) + 
     tm_fill() +
tm_shape(data %>% filter(category='cat1')) + 
     tm_polygons(varinterest) +

However, this is a bit slow (taking 5-7 minutes to render on a very strong computer). Mapping only the variable of interest (i.e. the second tm_shape(... takes only a few seconds.

I feel like I am missing something obvious that might speed up rendering - any advice?

I will work on a reproducible example and post my times.

Spacedman · Answer

Here's a reproducible example.  Get county shapes from USAboundaries:

> library(USAboundaries)
> data = us_counties()
> dim(data)
[1] 3220   13

add a category column with about 300 in each category, and a varinterest

> data$category=paste0("cat",sample(1:10,nrow(data),TRUE))
> table(data$category)

cat1 cat10  cat2  cat3  cat4  cat5  cat6  cat7  cat8  cat9 
  342   324   329   310   344   310   332   289   321   319 
> data$varinterest = runif(nrow(data))

Your tm_shape line looks a bit broken - you've got category='cat1' which needs a ==, no quotes round varinterest which are necessary, and a stray + sign on the end. Fixing those gives:

> tm_shape(data) + tm_fill() + tm_shape(data %>% filter(category=='cat1')) + tm_fill("varinterest")

which runs in seconds on my mediocre computer.

Avoid calling tm_shape() multiple times?

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