Show overall average on plots of single data

I have data from a survey from multiple locations (1 data set per location per year, each containing about 20-25 numerical scores from 0 to 5). This data is aggregated over topics (e.g. first 5 questions are Topic 1, questions 6-10 are Topic 2) so I can calculate the average score per topic, per location, per year.

I would like to show radar charts showing the values for one location and comparing it with the overall average; I thought about creating a fake data set, call it "Average" and always plot it together with the individual locations; I am new to PowerBi but I believe this is not the way to go and that I am missing an option to implement it that does not require fake data sets.

How do I show overall average when plotting single data sets?

Edit:
I was able to find an answer on the PowerBi forum: https://community.powerbi.com/t5/Desktop/Power-BI-Compare-Individual-vs-Average-data-per-month/td-p/355805, but do not understand how to create the "right" measures for it