Expressing Constraints for Data Display Specification: A Visual Approach

In this paper we introduce a constraint-based language that has a visual syntax, and allows for the declarative speciication of the display of data. Other features of the proposed language include: (1) simplicity and genericity of the basic constructs; (2) ability to specify a variety of displays (graphs, bar charts, pie charts, etc.); (3) compatibility with the object-oriented framework of the database language doo-dle. We provide the syntax and the semantics of the language, and examples of applications that demonstrate the expressiveness of our language. 1.1 Introduction Mappings between the data domain and the visual domain are commonly used for extracting information from the data by reasoning in the visual domain 3, 12]. For example, Venn diagrams are visual representations of abstract sets and of their inclusion relationships. Other diagrams are close to the concrete entities that they represent, such as transportation and communication networks. Bar charts, pie charts, and plot charts can facilitate the comprehension of large amounts of quantitative data 20]. In this paper we present a visual constraint-based language, called U-term language , to specify the geometric layout of pictures (e.g., as displayed on a computer screen), i.e., sets of graphical objects. Syntactically, a picture is a well-formed sentence in a visual language. Pictures in the U-term language are called U-terms, and are used to specify pictures on the computer screen. We have designed the U-term language to provide: A declarative and visual speciication of pictures with simple and generic constructs. The ability to specify a variety of pictures such as graphs, bar charts, and pie charts, using Cartesian or polar coordinates. Easy integration in an object-oriented framework. This paper does not address constraint satisfaction issues, which are covered elsewhere 8], and summarized in a later section. Previous work on data display speciication includes constraint-based languages that have a textual syntax, such as TRIP 13], which uses Prolog terms and functors