Defining a layout of diagram elements can be performed in response to user input that includes one or more declarative statements specifying conditional patterns based on attributes of diagram elements, wherein the conditional patterns define layouts of diagram elements. Implementation of the layouts is dependent on conditions defined in the declarative statements and one or more values of one or more of the attributes. Implementations can further include organizing the conditional patterns as a pattern definition, wherein the pattern definition is stored on a computer readable medium in such a way that the pattern definition is retrievable by an application program that uses the pattern definition to evaluate the conditional patterns using values of attributes of one or more diagram elements. Representations of the diagram elements can be displayed according to the layouts when conditions for implementing the layouts are satisfied.