“Underlying the concept of visualization is the idea that an observer can build a mental model, the visual attributes of which represent data attributes in a definable manner. This raises several questions: – What mental models most effectively carry various kinds of information ? – Which definable and recognizable visual attributes of these models are most useful for conveying specific information either independently or in conjunction with other attributes ? – How can we most effectively induce chosen mental models in the mind of an observer ? – How can we provide guidance on choosing appropriate models and their attributes to a human or automated display designer ?     Choosing the appropriate representation can provide the key to critical and comprehensive appreciation of the data, thus benefiting subsequent analysis, processing, or decision making.” [ROB91]