Below is a polygon filled map of London, United Kingdom.
Polygon maps are easy to make in Tableau provided you’re wanting to map US regions or very top level regions in Europe (country level for example). But if you want to make a post code level map in Tableau or map postal areas in Tableau then you’ll become a bit stuck.
Thankfully there is a “hack” that allows you to make your own custom maps that was highlighted by Craig Bloodworth at The Information Lab. The method works by plotting the longitude and latitude of every edge that makes up a polygon. Give Tableau the order that these should be connected (path id) and you’ll be able to draw a custom shaped polygon on a Tableau map. You can even add functionality like parameters to show different metrics on your area fill map.
One sticking point is the longitude and latitude of every UK borough isn’t readily available. Thankfully, the link above contains several Tableau data extracts that are free to download. Each one allows you to map UK data at different levels.
The example I shared at the start of the post shows how you can also map a second data series on top of the polygons. Using a dummy data set, I’ve shown how you might map incidents on top of London boroughs (just an example, obviously you could just shade each London borough according to incidents). The trick to achieving this result is to use a dual axis when dragging the longitude and latitude on to columns and rows for the second series.