Data records contain corresponding values of an attribute and are associated with location information. Hierarchical levels of transparency images are generated, where each of the transparency images includes corresponding pixels that represent the respective data records, and where the transparency images at different ones of the hierarchical levels correspond to different zoom levels of a geographic map. At least one of the transparency images at a dynamically-selected one of the hierarchical levels is overlaid over the geographic map according to which of the zoom levels is selected by a user in zooming of the geographic map, where the at least one transparency image is automatically selected without user input.