I have multiple layers in my map, each layer representing a different GP practice (doctor's). Each layer covers around 50 polygons with their boundaries being exactly the same in all layers. Every polygon has a count of patients of that medical practice who are sick. What I would like to do is create a web app that would let the user select GP practice 1 and see how many people are sick in Area A, and then select GP Practice 2 as well, and see how many people are ill in the same Area A summing GP 1 and GP 2 records (covering as many GP practice layers as the user wants). Ideally I am looking to portray that as a heat map. Is there a smart way to do this? Thank you!