Evaluation of symbol contrast in scatterplots

Symbols are frequently used to represent data objects in visualization. An appropriate contrast between symbols is a precondition that determines the efficiency of a visual analysis process. We study the contrast between different types of symbols in the context of scatterplots, based on user testing and a quantitative model for symbol contrast. In total, 32 different symbols were generated by using four sizes, two classes (polygon-and asterisk shaped), and four categories of rotational symmetry; and used three different tasks. From the user test results an internal separation space is established for the symbol types under study. In this space, every symbol is represented by a point, and the visual contrasts defined by task performance between the symbols are represented by the distances between the points. The positions of the points in the space, obtained by multidimensional scaling (MDS), reveal the effects of different visual feature scales. Also, larger distances imply better symbol separation for visual tasks, and therefore indicate appropriate choices for symbols. The resulting configurations are discussed, and a number of patterns in the relation between properties of the symbols and the resulting contrast are identified. In short we found that the size effect in the space is not linear and more dominant than shape effect.