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This paper extends previous work on focus+context visualizations of tree-structured data, introducing an efficient, space-constrained, multi-focal tree layout algorithm ("TreeBlock") and techniques at both the system and interactive levels for dealing with scale. These contributions are realized in a new version of the Degree-Of-Interest Tree browser, supporting real-time interactive visualization and exploration of data sets containing on the order of a million nodes.
Heer et al. (Tue,) studied this question.