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Tree structured data and XML: visualizing and transforming data with SVG and XSLT.

Article Excerpt
XML is the appropriate format for semistructured data, that is, data with a natural tree structure. Trees are a special form of graphs, and a dialect of XML called GraphML now exists that provides a standard set of tags for describing them. To visualize or draw the graph we can use another XML dialect called SVG, or Scalable Vector Graphics.

SVG is a language for describing two-dimensional graphics and graphical applications and is a dialect of XML. In this article we discuss the representation of tree-structured data using XML and GraphML. We visualize the data using SVG and transform the data between the various XML documents using Extensible Stylesheet Language Transformations (XSLT).

Introduction

A graph is simply a collection of nodes and edges and may be directed or undirected. In the case of a directed graph, an arrow connects the source to the target; for an undirected graph there is no arrow, only a line. A tree is a special case of a graph: there are no closed loops or circuits but all the nodes are connected. XML documents satisfy this definition and thus exhibit a natural tree structure. Examples of trees include decision trees, concept maps, and the structure of hydrocarbon molecules.

The structure of a tree is shown in Figure 1. The rectangles are the nodes and the edges are the lines connecting the nodes. Node A is a source and targets nodes B, C, and D; similarly, source node D targets nodes E and E

[FIGURE 1 OMITTED]

The tree in Figure 1 can be represent ed in XML by a series of tags as shown in Figure 2. The nodes of the tree become tags in XML.

[FIGURE 2 OMITTED]

Alternatively, we can use a standard format like GraphML to illustrate a tree. In GraphML the tree shown in Figure 1 becomes...