Building a choropleth map

Choropleth map is a thematic map, in other words, a specially designed map not a general purpose one, which is designed to show measurement of statistical variable on the map using different color shades or patterns; or sometimes referred as geographic heat-map in simpler terms. We have already seen in the previous two recipes that geographic projection in D3 consists of a group of svg:path elements, therefore, they can be manipulated as any other svg elements including coloring. We will explore this feature in geo-projection and implement a Choropleth map in this recipe.

Getting ready

Open your local copy of the following file in your web browser hosted on your local HTTP server:

https://github.com/NickQiZhu/d3-cookbook/blob/master/src/chapter12/choropleth.html

How to do it...

In a choropleth map different geographic regions are colored according to their corresponding variables, in this case based on 2008 unemployment rate in US by county. Now, let's see how...