Check the below video on how to overlay new geographical role over you maps, or add it to an existing hierarchy:
Thanks for the link. I've been through it, but I'm not sure any of the solutions apply.
I am adding a new geographic role (the postcodes), but it doesn't make sense to add them to the NUTS3 areas. The postcodes refer to institutions - and each will fall into a NUTS3 area, but there is not a one-to-one correlation.
Also, I don't know whether this will affect the solution in anyway, but I need to be able to filter the institutions so that only one is displayed at a time (whilst always displaying all of the NUTS3 polygons).
It sounds to me like you want to use Tableau Geocoding polygons + a text file with lat/lon pairs. You should be able to combine these in a dual-axis map. I have written up some instructions on ways that you can combine different geographic data sources into dual-axis maps in this blog post: Dual Axis Mapping - Many Ways
Take a look at the spatial file + csv section. I think that it will help explain how you can combine these two datasets. Let me know if you have problems and I'll see if I can work up an example workbook for you.
That did the trick, thank you very much. That is a very useful article you've written there!