Creating choropleth maps in R with the darkest colour at the top

I’ve just been through the process of contributing to the source code of a package in R (in a very small way) so here’s a short piece on how easy it was, and why anyone can do it! I originally wrote this post in August last year, but waited to post it until the new version of maptools was released. I missed this (we are now at 0.8-39!) and have only just rediscovered this post. It’s all still relevant though!

I have been using the Maptools library extensively in my use of R as a GIS, as well as in my teaching material (hosted at https://github.com/nickbearman/intro-r-spatial-analysis). The default plot order in the legend is to have the darkest colour at the bottom of the legend, and the lightest colour at the top. This was just something I accepted, and to be honest, never really thought about before.

I recently delivered a training course on R to some staff at the ONS (Office for National Statistics, England & Wales) and they said that their best practice guidelines are to have the darkest colour at the top of the legend. They asked me how to do this, which I didn’t know!

After some fiddling about with an R script, I created a version which worked for them. I then thought it might be useful to integrate this into the Maptools library, and emailed the package author, Roger Bivand. He was very helpful, and I added the additional code to the sourcefiles for Maptools. These are now avaliable in version 0.8-37 (or later), which has recently be released. Running update.packages(“maptools”) should get you the new version.

To reverse the colours is a simple matter of changing the legend code in two places. Using the example from the helpfile, the original line:

legend(x=c(5.8, 7.1), y=c(13, 14.5), legend=leglabs(brks), fill=colours, bty="n")

The revised line:

legend(x=c(5.8, 7.1), y=c(13, 14.5), legend=leglabs(brks, reverse = TRUE), fill=rev(colours), bty="n")

To give you some nice visual examples:

Rplot Rplot_reverse

Or for those of you who have attended my R course:

normal-order reverse-order

The file I updated is at https://r-forge.r-project.org/scm/viewvc.php/pkg/R/colslegs.R?view=markup&root=maptools (this link shows the changes), and I also updated the helpfile. If you’ve done some R scripting, then it is not too difficult to do. Any questions, please post them here. Good luck!

 

2 thoughts on “Creating choropleth maps in R with the darkest colour at the top

  1. Isi

    Hi Nick,

    This is really useful thank you! On a slightly unrelated note, do you know how I would go about adding another legend label to the legend. My data has a lot of N/As and when I plot this it just leaves white gaps in the map but I would like to be able to label that this missing data.

    It would be much appreciated if you could help!

    Thank you!

    Reply
    1. Nick Post author

      Hi Isi,
      Thanks very much – I’m glad you found it useful. In terms of adding a extra legend, I think you can just call the legend() function again and say legend(x, y, “Text you want”, fill = X, col = X, …) to add the extra information. The help (?legend) has some info and this post has some info on the basic legend function. Have a go and let me know how you get on!
      Good luck!
      Nick.

      Reply

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