CODEWITHSUNDEEP
rggplot2choropleth
Tyler Rinker
Tyler Rinker

Reputation: 110024

geom_map all subregions the same color

I'm attempting to move over from older ways of mapping data to a choropleth now that ggplot2 has the geom_map. An example is seen on pp. 10-11 (HERE).

I'm attempting to do this with a data set I've created a choropleth from in the past just not with ggplot's new geom_map. Here's my attempt that I think is like Hadely's example but everything is the same color.

The data set and code:

#loads 2 data frames: ny and cad from my drop box
load(url("http://dl.dropbox.com/u/61803503/MAPPING.RData"))
library(ggplot2)

ggplot(cad, aes(map_id = subregion)) +
    geom_map(aes(fill = Math_Pass_Rate), map = ny) +
    expand_limits(x = ny$long, y = ny$lat) +
    guides(fill = guide_colorbar(colours = topo.colors(10))) +
    opts(legend.position = "top")

Why is it showing up as the same color?

Additional information from @PaulHiemstra

I've puzzling a bit on it, and could not get a good result. However, I also wonder why the example from the ggplot2 pdf you link to works.

This code produces a correct choropleth map.

crimes <- data.frame(state = tolower(rownames(USArrests)), USArrests)
states_map <- map_data("state")
ggplot(crimes, aes(map_id = state)) +
   geom_map(aes(fill = Murder), map = states_map) +
   expand_limits(x = states_map$long, y = states_map$lat) +
   guides(fill = guide_colorbar(colours = topo.colors(10))) +
   opts(legend.position = "top")

One would expect that by using map_id = state, that a link was made between a column in states_map (the polygons) and a column in crimes (Murder). crimes contains a state column:

> head(crimes)
                state Murder Assault UrbanPop Rape
Alabama       alabama   13.2     236       58 21.2
Alaska         alaska   10.0     263       48 44.5
Arizona       arizona    8.1     294       80 31.0
Arkansas     arkansas    8.8     190       50 19.5
California california    9.0     276       91 40.6
Colorado     colorado    7.9     204       78 38.7

but states_map does not:

> head(states_map)
       long      lat group order  region subregion
1 -87.46201 30.38968     1     1 alabama      <NA>
2 -87.48493 30.37249     1     2 alabama      <NA>
3 -87.52503 30.37249     1     3 alabama      <NA>
4 -87.53076 30.33239     1     4 alabama      <NA>
5 -87.57087 30.32665     1     5 alabama      <NA>
6 -87.58806 30.32665     1     6 alabama      <NA>

So in the link between the polygons and the data, some black magic seems to be happening. This might also explain the problems @TylerRinker has.

Upvotes: 3

Views: 1529

Answers (1)

Paul Hiemstra
Paul Hiemstra

Reputation: 60984

This is documented behavior of geom_map. geom_map always draws the region variable (or alternatively id) from states_map. This is confirmed by the following. Running:

ny$region = ny$subregion

puts the subregion names into the region column. Now plotting leads to the correct image:

enter image description here

So, geom_map uses the region or id.

Upvotes: 5

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