4 Hierarchical data

Many data sets contain within them – either explicitly or implicitly – a set of structural relations between different parts of the data set.

One common way of structuring data is in the form of a hierarchy, or ‘family tree’. Typical examples are organisational charts and library classification schemes.

Hierarchical diagrams are also widely used as the basis of mind-mapping tools, where ‘child’ ideas are developed leading off from a central core topic. A mind-mapping tool can provide a very good way of helping you ‘unpack’ or explore an idea.

One of the problems with displaying hierarchies is that they can get very large – and hard to display – very quickly.

There are several ways around this problem. For example, an interactive visualisation can ‘collapse’ each branch of the tree, hiding the sub-branches until you want to see them. In this sense, hierarchical organisations can also be thought of as containing sets of ‘boxes within boxes’.

You may already be familiar with this sort of approach from your computer – many file managers offer a hierarchical visualisation of file organisation through ‘nested’ folders which you can open up or collapse as you wish. Figure 11 shows an example of this.

Figure 11 An example of a hierarchical folder structure with some of the folders opened to show their contents

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This figure shows a set of folder icons with some of the files having the contents showing as well. At the top left the highest level file is shown. To the left of each folder symbol is an arrowhead and to the right of the folder symbol is the folder’s name. The top folder is called ‘jit’. The arrowhead next to the jit file points downwards indicating that the contents of that file are shown. In the next row down and set 1 jump to the right is another arrowhead with a folder symbol next to it and name next to that. Whenever an arrowhead points downwards the next level of folder or a file is shown underneath. Further down the figure, folders are shown with the arrowhead pointing to the right. In this case no further levels of folder are shown. At levels where there are no further folders, files such as main.css can be seen, with appropriate icons.

Figure 11 An example of a hierarchical folder structure with some of the folders opened to show their contents