Thought I'd put my answer below, the tutorial you followed just takes you through the basics, for a full documentation see https://appstore.home.mendix.com/link/app/1712/ under the documentation tab
The specific part of that guide that applies to you is:
"Data Point Entity
The reference (set) that connects the Data Set Entity to the Data Point Entity. This entity contains the values for each point in the dataset. This is a reference set for the Line, Bar, StackedBar and Radar charts. It is a reference for the Pie, Doughnut, DoubleDoughnut and Polar charts"
So you would need a Many to One relationship as well which gets used in the pie chart.
Hope this helps