Hierarchical Aggregation for Information Visualization: Overview, Techniques, and Design Guidelines <s> Tree Visualizations <s> In many contexts, humans often represent their own “neighborhood” in great detail, yet only major landmarks further away. This suggests that such views (“fisheye views”) might be useful for the computer display of large information structures like programs, data bases, online text, etc. This paper explores fisheye views presenting, in turn, naturalistic studies, a general formalism, a specific instantiation, a resulting computer program, example displays and an evaluation. <s> BIB001 </s> Hierarchical Aggregation for Information Visualization: Overview, Techniques, and Design Guidelines <s> Tree Visualizations <s> The traditional approach to representing tree structures is as a rooted, directed graph with the root node at the top of the page and children nodes below the parent node with lines connecting them (Figure 1). Knuth (1968, p. 305-313) has a long discussion about this standard representation, especially why the root is at the top and he offers several alternatives including brief mention of a space-filling approach. However, the remainder of his presentation and most other discussions of trees focus on various node and edge representations. By contrast, this paper deals with a two-dimensional (2-d) space-filling approach in which each node is a rectangle whose area is proportional to some attribute such as node size. <s> BIB002 </s> Hierarchical Aggregation for Information Visualization: Overview, Techniques, and Design Guidelines <s> Tree Visualizations <s> Existing information visualization techniques are usually limited to the display of a few thousand items. This article describes new interactive techniques capable of handling a million items (effectively visible and manageable on screen). We evaluate the use of hardware-based techniques available with newer graphics cards, as well as new animation techniques and non-standard graphical features such as stereovision and overlap count. These techniques have been applied to two popular information visualizations: treemaps and scatter plot diagrams; but are generic enough to be applied to other 2D representations as well. <s> BIB003 </s> Hierarchical Aggregation for Information Visualization: Overview, Techniques, and Design Guidelines <s> Tree Visualizations <s> We present a novel tree browser that builds on the conventional node link tree diagrams. It adds dynamic rescaling of branches of the tree to best fit the available screen space, optimized camera movement, and the use of preview icons summarizing the topology of the branches that cannot be expanded. In addition, it includes integrated search and filter functions. This paper reflects on the evolution of the design and highlights the principles that emerged from it. A controlled experiment showed benefits for navigation to already previously visited nodes and estimation of overall tree topology. <s> BIB004 </s> Hierarchical Aggregation for Information Visualization: Overview, Techniques, and Design Guidelines <s> Tree Visualizations <s> This paper proposes Degree-of-Interest trees. These trees use degree-of-interest calculations and focus+context visualization methods, together with bounding constraints, to fit within pre-established bounds. The method is an instance of an emerging "attention-reactive" user interface whose components are designed to snap together in bounded spaces. <s> BIB005 </s> Hierarchical Aggregation for Information Visualization: Overview, Techniques, and Design Guidelines <s> Tree Visualizations <s> An equity mutual fund is a financial instrument that invests in a set of stocks. Any two different funds may partially invest in some of the same stocks, thus overlap is common. Portfolio diversification aims at spreading an investment over many different stocks in search of greater returns. Helping people with portfolio diversification is challenging because it requires informing them about both their current portfolio of stocks held through funds and the other stocks in the market not invested in yet. Current stock/fund visualization systems either waste screen real estate and visualization of all data points. We have developed a system called FundExplorer that implements a distorted treemap to visualize both the amount of money invested in a person's fund portfolio and the context of remaining market stocks. The FundExplorer system enables people to interactively explore diversification possibilities with their portfolios. <s> BIB006 </s> Hierarchical Aggregation for Information Visualization: Overview, Techniques, and Design Guidelines <s> Tree Visualizations <s> Treemaps provide an interesting solution for representing hierarchical data. However, most studies have mainly focused on layout algorithms and paid limited attention to the interaction with treemaps. This makes it difficult to explore large data sets and to get access to details, especially to those related to the leaves of the trees. We propose the notion of zoomable treemaps (ZTMs), an hybridization between treemaps and zoomable user interfaces that facilitates the navigation in large hierarchical data sets. By providing a consistent set of interaction techniques, ZTMs make it possible for users to browse through very large data sets (e.g., 700,000 nodes dispatched amongst 13 levels). These techniques use the structure of the displayed data to guide the interaction and provide a way to improve interactive navigation in treemaps. <s> BIB007
When the data structure is a tree, it is generally used as its own aggregation hierarchy. For this reason, we discuss all tree visualizations together in one section. Using the tree structure itself as the aggregation hierarchy is problematic when the tree is unbalanced or when it is either very deep, such as phylogenetic trees, or very wide, such as the Java SDK class hierarchy. In this case, the aggregation hierarchy typically merges or splits some of the original tree nodes but remains congruent with the original tree structure. Treemaps BIB002 visualize tree structures through enclosure using a space-filling layout algorithm based on recursive subdivision of space. The enclosure feature of treemap nodes conveys the extents of underlying nodes in the hierarchy. Furthermore, the surface of an internal tree node serving as a visual aggregate could conceivably be used to show additional information about the node's children. However, because the size of visual entities depends on a specific data attribute, some branches of a tree may disappear (i.e., be allocated display size less than a pixel) BIB002 , a problem addressed by context treemaps BIB006 that allocate space even to zero-sized nodes. Fekete and Plaisant BIB003 give a number of technical solutions for visualizing treemaps on the order of 10 6 items, cutting off rendering for data items that are smaller than one pixel in size. The zoomable treemaps BIB007 introduced by Blanch and Lecolinet combine many of the interaction techniques described in this paper with the use of treemaps as a multiscale space for navigation. Their work is instructive in how to extend an existing visualization technique with support for aggregation and navigation in the aggregate hierarchy. SpaceTrees BIB004 use standard node-link representations of trees, but support interactive expansion and contraction of tree branches as well as screen-optimized dynamic layout of nodes and automatic camera movement to fit the current display. The technique supports many of the general interaction techniques for hierarchical aggregation described in this paper, including local aggregation control, roll-up, and drill-down. Visual aggregates of unexpanded branches are shown using thumbnails that indicate the underlying node topology, or as simple triangle glyphs that show the count and depth of the contracted subtree. Degree-of-Interest trees BIB005 provide a focus+context BIB001 view of node-link tree representations that are optimized for the screen. As with SpaceTrees, DOITrees define several different visual representations for aggregated subtrees, including elided and glyph representations of their node count and depth. 