SSRS How to Use Aggregate on Pie Chart?

Question

I want to use the Max() function to only return the highest date within the ShipDate field for each order. SSRS 2012 doesn't allow you to use an aggreagte in a calculated field and I would normally use it as an expression in a table, but I also need it within a Pie Chart. Now I can't see how you use an expression in a Pie Chart, it only lets me select a field created within the DataSet. What would be the best way to get around this issue?