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I'm trying to make a choropleth map of counties in the US. My values have a large range, and are highly 0 inflated, so I would like to create custom bins for the legend and coloring. Ideally this would be using ggplot. I would also like to keep a standard structure for the legend so that I can use the same legend for multiple choropleths with different data.

The legend might be something 0, 1-20, 21-50, 51-150, 151-300, 301-800, with each "bin" having its color derived from Spectral or some other continuous color palette.

scale_color_steps(), scale_fill_brewer() and other similar functions, even with breaks and limits or n.breaks, don't seem to work. Any ideas?

library(choroplethr)
library(choroplethrMaps)
library(ggplot2)
data(county.regions)

df <- tibble(region = county.regions$region, value = c(rep(0,2000), rep(1,1000), sample(2:800, 143)))  

county_choropleth(df) +
  scale_colour_steps(palette='Spectral', n.breaks = 8)
Alex Krohn
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    What do you mean by "custom bin size"? Do you mean "custom bins"? – Edward Jun 25 '20 at 22:57
  • So what exactly is the desired result here? – MrFlick Jun 25 '20 at 23:02
  • @Edward The goal would be to have a continuous legend with breaks or bins that I specify, e.g. 0, 1-20, 21-50, 51-150, 151-300, 301-800, with each "bin" having its color derived from Spectral or some other continuous color palette. I'll edit the original post too. – Alex Krohn Jun 25 '20 at 23:57
  • Thanks for using Choroplethr. I'll echo the other commenters. It would be nice if your question had a very clear desired end result, so we could be sure that our answer would be accepted. – Ari Jun 26 '20 at 00:27

1 Answers1

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The simple solution to achive your desired result is to set num_colors = 1 in country_choropleth.

From the docs of country_choropleth:

num_colors The number of colors to use on the map. A value of 0 uses a divergent scale (useful for visualizing negative and positive numbers), A value of 1 uses a continuous scale (useful for visualizing outliers), and a value in [2, 9] will use that many quantiles.

By default num_colors = 7 which means that the continuous data gets discretized so that adding a continuous fill scale throws an Error: Binned scales only support continuous data. Setting num_colors = 1 the data don't gets binned and we can apply a custom continuous fill scale:

library(choroplethr)
library(choroplethrMaps)
library(ggplot2)
data(county.regions)

set.seed(42)

df <- tibble::tibble(region = county.regions$region, value = runif(3143, 0, 800))  

breaks <- c(0, 20, 50, 150, 300, 800)

county_choropleth(df, num_colors = 1) +
  scale_fill_stepsn(colors = RColorBrewer::brewer.pal(5, "Spectral"), breaks = breaks)


county_choropleth(df, num_colors = 1) +
  scale_fill_viridis_b(breaks = breaks)

stefan
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  • Please note that there is a problem with this answer: using ggplot2's `+` operator does not affect the scale used for AK or HI. To learn how to fix this, please see e.g. the last part of my answer to this question: https://stackoverflow.com/questions/61771543/assigning-colors-to-table-values-using-choroplethr/61787021#61787021 – Ari Jun 26 '20 at 17:43