Design Principles and Decisions
Creating a visualisation involves making many design decisions from the style and colour of individual graphic objects to the sizing and positioning of graphic elements (graph, map, legend, scalebar, etc.) on a layout, which may be a static image or interactive interface. Design principles and guidelines are collections of rules or best-practice advice derived from quantitative and qualitative evaluation of alternative visualisation approaches that should be considered when composing any graphic product.
When making a graphic, you should identify where design decisions are being made, evaluate design principles, and then make reasoned decisions. This is particularly important for quantitative graphics, as design decisions determine the patterns shown and influence how the graphic is read and decoded by the viewer. A good example of this is selecting a classification and color scheme for a choropleth map where the classification scheme determines the pattern.
Principles and guidelines are helpful and sometimes essential for enabling ‘good design’; however, experimental praxis (‘doing’) is required to test ideas, fine-tune products, and to innovate.
Levels of design
Data graphic design requires decisions to be made at different graphic ‘levels’ from low-level decisions relating to the design of individual graphics (symbol style, size, color, and font selection) to high-level decisions concerned with how symbols interact within a graphic and the arrangement of graphic elements on a layout.
Design guides may start with either high- or low-level design, or may present a ‘buffet’ of topics that can be dipped into. Each has its merits; design principles at different levels interact. For example, a change in a symbol may shift it from the background to the foreground, and the desire to move a symbol to the foreground is enacted by increasing the visual prominence of the symbol by changing its design or the design of background graphics.
High-level design
High-level design is concerned with graphic compositions as a whole, while low-level design is concerned with the basic graphic ‘marks’ and their local juxtaposition that combine to create a ‘whole,’ which may be the entire layout, a figure, or other hierarchical groupings of graphic elements. High-level design principles are difficult to categorize as they apply to some or all levels of organization and some or all forms of visualization.
Low-level design
Low-level design principles relate to the graphic symbols and text employed that must be visually discernible. Differentiation of symbols is implemented by employing graphic dimensions (also known as retinal dimensions) to design symbols that encode one or more attribute values such as shape, size, color, and orientation. Similarly, text dimensions of font, size, color, and form (regular/bold/italic), etc. The graphic dimensions that are available and whether they produce ‘good’ outcomes depend on the type of mark (point, line, area, text).