  Data analysis often involves the comparison of complex objects. With the ever
increasing amounts and complexity of data, the demand for systems to help with
these comparisons is also growing. Increasingly, information visualization
tools support such comparisons explicitly, beyond simply allowing a viewer to
examine each object individually. In this paper, we argue that the design of
information visualizations of complex objects can, and should, be studied in
general, that is independently of what those objects are. As a first step in
developing this general understanding of comparison, we propose a general
taxonomy of visual designs for comparison that groups designs into three basic
categories, which can be combined. To clarify the taxonomy and validate its
completeness, we provide a survey of work in information visualization related
to comparison. Although we find a great diversity of systems and approaches, we
see that all designs are assembled from the building blocks of juxtaposition,
superposition and explicit encodings. This initial exploration shows the power
of our model, and suggests future challenges in developing a general
understanding of comparative visualization and facilitating the development of
more comparative visualization tools.
