[R-sig-Geo] Labelling a fortified GADM map plotted with ggplot and geom_map

[Roger Bivand]

On Mon, 19 Mar 2018, Kenneth Dyson wrote:

> Reffered here from the R-Help mailing list:
>
> I am having trouble getting data labels to display over the provinces in 
> a GADM map of Canada. Specifically, I need the variable "Number" from 
> the data set "by_province", grouped by "region", to appear on the 
> corresponding regions of the map.

You are making life unnecessarily difficult. Use the tmap package instead, 
see for example:

https://www.johnmackintosh.com/2017-09-01-easy-maps-with-tmap/

using the sf representation and forgetting ggplot2 (the development 
version supports geom_sf, but hasn't been released).

Roger

>
> The data set "by_province" looks like this:
>
> long      lat          order  hole piece                   region                group     Number
> -110.37074 60.00006       1 FALSE     1            Alberta            Alberta.1    132
> -110.36250 60.00006       2 FALSE     1            Alberta            Alberta.1    132
> -110.35103 60.00006       3 FALSE     1            Alberta            Alberta.1    132
>
> and the data set "tract" is the map data without the "Number" variable.
>
> I looked into working this out from by researching the geom_map function here: http://ggplot2.tidyverse.org/reference/geom_map.html
>
> my code looks like this:
> # get the raw map data
> can_map <- getData('GADM', country = "CAN", level = 1) # download map level with provinces
> tract <- fortify(can_map, region = "NAME_1") # transforms data from shapefiles into a dataframe that ggplot can understand, from http://www.kevjohnson.org/making-maps-in-r/
>
> # create subsets of the kcj data if needed, for example by year
> kids_data_2017 <- subset(kids_data, year == 2017) # data for the year 2017
> kids_data_2018 <- subset(kids_data, year == 2018) # data for the year 2018
>
> # extract the needed data
> kids_province_data <- data.table::data.table(kids_data_2017$Province_Territory__c, kids_data_2017$Number_of_kids__c)
> names(kids_province_data)[1] <- "Province"
> names(kids_province_data)[2] <- "Number"
>
> # sum the data by province
> kids_province_sums <- aggregate(.~Province, data = kids_province_data, sum)
>
> # join the data to the map
> names(tract)[6] <- "region"
> names(kids_province_sums)[1] <- "region"
> by_province <- left_join(tract, kids_province_sums)
>
> # create the data map
> kids_map <- ggplot(by_province, aes(map_id = region)) +              #plots the map in by_province separating by region
>            geom_map(aes(fill = Number),                             #generates aestheticsa for the plot
>                     map = tract,                                        #takes the data from tract
>                     color = "#ffffff",                                  #makes the color of the borders between regions white
>                     size = 0.15) +                                      #sets the thickness of the boarder lines
>            coord_map("polyconic") +                                 #sets the coordinates to polyconic (rounded lat and long)
>            scale_fill_gradient(name = "Children Reached",           #sets the gradient of the value scale: names the scale
>                                low = grey_2,                            #color of the low end
>                                high = orange_1) +                       #color of the high end
>            expand_limits(x = tract$long,                            #ensure limits include all values for all plots
>                          y = tract$lat) +
>            labs(x = NULL,                                           #add labels, no x title
>                 y = NULL,                                               #no y title
>                 title = "Number of Children Reached by Province",       #map title
>                 subtitle = "2017") +                                    #map subtitle
>            geom_text(data = by_province,                            #add a text layer
>                      aes(long,                                          #aethetics for the text, x axis
>                          lat,                                               #y axis
>                          label = Number,                                    #the value to display
>                          size=3)) +                                         #size of the text
>            theme(axis.ticks = element_blank(),                      #theme of the graph, no axis ticks
>                  axis.text = element_blank(),                           #no axis text
>                  panel.background = element_blank(),                    #blank background
>                  panel.border = element_blank())                        #blank border
>
> # save as png
> ggsave(kids_map, file = "kids_map.png", width = 6, height = 4.5, type = "cairo-png”)
>
>
> I have asked this question on stack overflow, and was refered to this answer:
> https://stackoverflow.com/questions/9441436/ggplot-centered-names-on-a-map
>
> The solution there did not fix my problem, though it did get me closer.
> The solution on that post is using a single vector of labels.
>
> My post on stackoverflow has images of my output:
> https://stackoverflow.com/questions/49118323/labelling-a-map-plotted-with-geom-map
>
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>
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-- 
Roger Bivand
Department of Economics, Norwegian School of Economics,
Helleveien 30, N-5045 Bergen, Norway.
voice: +47 55 95 93 55; e-mail: Roger.Bivand at nhh.no
http://orcid.org/0000-0003-2392-6140
https://scholar.google.no/citations?user=AWeghB0AAAAJ&hl=en