If you use SqlDataSource and follow the documentation step by step (all requirements are satisfied), then you create the aggregate function as shown in the example (may want to change the logic there, which parameters should be selected, as the example selects all).
And then the expression would be Value = AllValues(Fields.YourField). See attached screenshot for more info.
If you are using objectDataSource then you can bind to custom user function that returns an array of data.
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