In an example, a method may include generating a semantic layout of a visualization based on a contract associated with a visualization type. The method may also include receiving input effective to alter a set of one or more graphical objects to display for each of some but not all data objects of a data source from a default set of one or more graphical objects to a custom set of one or more graphical objects. The method may also include generating a concrete layout of the visualization that specifies the set of one or more graphical objects to display for each of the data objects, including the default set for a first data object and the custom set for a second data object. The method also includes displaying the visualization based on the contract, the semantic layout, and the concrete layout, the visualization having the visualization type.