It is really difficult to grasp the significance of lots of big numbers. It is even trickier when the numbers are organised in a hierarchy. For example, yesterday afternoon Bill English, the Minister of Finance, delivered his third budget, outlining the nation’s revenues and expenses. The budget includes details such as how the government plans to spend $21 billion dollars on social development in the coming year of which $9.5 billion will be spent on superannuation, almost $1.9 billion on the domestic purposes benefit, nearly $1.6 billion on accommodation assistance… and I’m already lost.

Treemapping is a technique for visually comparing groupings of numbers. A treemap represents a hierarchy of numeric values as a set of nested shapes – usually rectangles. I’m fond of this technique for several reasons, not least because it plays on existing associations. Big rectangles represent big numbers. Rectangles inside other rectangles indicate that one thing is part of another thing. Rectangles can even be shaded to depict an additional variable, for example, an increase or decrease over time.

Last night, with the assistance of Keith Ng, I created an interactive treemap of the New Zealand budget. It is available here. I encourage you to have a play with it. Unfortunately, given the large volume of data that needs to be processed and rendered, the visualisation struggles on every browser except Google Chrome. If you have Chrome on your computer, I highly recommend that you use it to view the visualisation.

Treemap of NZ Budget 2011

There are two levels to the treemap. The top level shows the breakdown of various expenditure areas. The bigger the rectangle, the more money is being spent. Green rectangles indicate that spending has significantly increased since the last budget; red rectangles indicate a decrease. When you click on any spending area, the visualisation will dive into that area and show a detailed breakdown.

Breakdown of education spending.

If you are interested in learning more about treemapping, I suggest you read Ben Shneiderman’s account of how he developed this class of visualisation and Thomas Kerwin’s survey of treemap techniques. Wikipedia has handy list of software if you want to create your own treemaps.