The Pie Chart control requires you to provide data that already calculates the numbers you would like to plot. In this case you would need to do a "group by" Product and perform the calculations on them and then provide that to the Pie Chart.
Here is an example on how you could add all the evaluations by product. Note that in this case I'm "hard-coding" a collection that attempts to match the schema you showed in your screenshot called "YourData" but really the only important line is the one creating the "ChartData" below.
ClearCollect(YourData,
{CustomerName: "Al", Product: "Hat", Evaluation: 1},
{CustomerName: "Brad", Product: "Hat", Evaluation: 3},
{CustomerName: "Carly", Product: "Jacket", Evaluation: 2},
{CustomerName: "David", Product: "Shoes", Evaluation: 2},
{CustomerName: "Edward", Product: "Hat", Evaluation: 2},
{CustomerName: "Frederic", Product: "Hat", Evaluation: 1}
);
ClearCollect(ChartData, AddColumns(GroupBy(YourData, "Product", "CustomerName"), "Total", Sum(CustomerName, Evaluation)))
The only important line is where it GroupBy "Product" and then does a "Sum" of all the evaluations of the customers that rated that Product. You can then bind this to your Chart Items property and should work.
If instead you wanted the average evaluation for each product you can just switch to use the Average function,
ClearCollect(ChartData, AddColumns(GroupBy(YourData, "Product", "CustomerName"), "Total", Average(CustomerName, Evaluation)))
Or if you wanted to render the "count of customers" that rated each product, use CountRows:
ClearCollect(ChartData, AddColumns(GroupBy(YourData, "Product", "CustomerName"), "Total", CountRows(CustomerName)))
Hope this helps. To repro this, just copy the formula on the OnVisible of the first screen (you might need to add a new screen and navigate to it on the designer and come back so that the OnVisible is triggered to see the results), optionally you could add it to a "OnSelect" of a button that you can easily trigger.
Here is the average pie chart:
