How to create a choropleth map using plot_geo()?

Question

I am trying to plot a choropleth map in R and found some very promising code using the function plot_geo() here: https://plot.ly/r/choropleth-maps/. Unfortunately I could not find out how to adapt the code to my problem and I couldn’t find helpful explanations on the internet.

Here's the original code:

# specify some map projection/options
g <- list(
  scope = 'usa',
  projection = list(type = 'albers usa'),
  showlakes = TRUE,
  lakecolor = toRGB('white')
)

plot_geo(df, locationmode = 'USA-states') %>%
  add_trace(
    z = ~total.exports, text = ~hover, locations = ~code,
    color = ~total.exports, colors = 'Purples'
  ) %>%
  colorbar(title = "Millions USD") %>%
  layout(
    title = '2011 US Agriculture Exports by State<br>(Hover for breakdown)',
    geo = g
  )

The data I am using is a numeric value, determining the colour to fill a country, and the two-digit ISO code of all European countries. So since I want to create a map of Europe, I set scope = 'europe' but I don’t understand what projection = list(type = 'albers usa') does and what I need to specify instead. I just dropped it thus.

g <- list(
  scope = 'europe')

I tried running the following code but the resulting map is “empty”, i.e. there is no colouring of any country.

plot_geo(data) %>%
  add_trace(
    z = ~value, locations = ~LAND_ISO,
    color = ~value, colors = 'Purples'
  ) %>%
  colorbar(title = "") %>%
  layout(geo = g
  )

empty map using plot_geo

My first guess was that maybe proving the ISO codes is not the proper way to specify the locationsargument. If so, what is required instead? Or do I need to set the locationmodeargument used in the code above? What would that be for Europe?

Can anyone help me with this? Thank you!

Answer 1

Your code seems to work for me. I created a dataframe as shown below and passed it to the code you provided above and the output I recieved was:

I expect that the issue is with the formatting of the data frame you pass to your code, check that the ISO-3 codes are strings and that the values are numeric etc.

library(plotly)

#Create dataframe with toy data:
  LAND_ISO <- c("AUT","BEL","BGR","HRV","CYP","CZE","DNK","EST","FIN","FRA","DEU","GRC","HUN","IRL","ITA","LVA","LTU","LUX","MLT","NLD","POL","PRT","ROU","SVK","SVN","ESP","SWE","GBR")
  value <- runif(length(LAND_ISO), 1, 10)

  data <- data.frame(LAND_ISO, value)

# Run your code:
g <- list(
  scope = 'europe')

plot_geo(data) %>%
  add_trace(
    z = ~value, locations = ~LAND_ISO,
    color = ~value, colors = 'Purples'
  ) %>%
  colorbar(title = "") %>%
  layout(geo = g
  )

FYI I believe

projection = list(type = 'albers usa')

just selects the projection method used to create a 2D map from a 3D globe - See https://en.wikipedia.org/wiki/Map_projection for more details.