Animating the difference between seats and votes in first past the post elections

9 May 2017 07:44 GMT

I've been playing with animated treemaps. This treemap illustrates the difference between the number of seats and votes won by political parties in each nation and region of Great Britain at the 2015 General Election. Northern Ireland is not shown as NI has its own distinct political parties, so comparisons with other parts of the UK are less meaningful.

I wanted to try presenting the data in this way because it helps address an interesting question: what is the role of animation in data visualisation? I've been thinking about this question ever since posting an animated uncertainty chart a few years ago.

Animation is superficially appealing because the eye loves motion, and interfaces that respond to user input feel alive. But sometimes animations in visualisations add little if anything to the reader's understanding of the data. They can be like animated transitions in PowerPoint, which tend to communicate that the presenter has spent more time thinking about the style of their presentation than the content.

I think animations can be valuable when they help show something about the data. The extent of change in a transition can be a dimension along which comparisons can be made. In this case, you could show two treemaps side by side (one for seats and one for votes) but the eye would have to move between the two to find the same region and make comparisons. Here you can hold the eye still and observe the magnitude of the transition from one state to the other. That comparison is meaningful in this context because in a perfectly proportional electoral system there would be no difference at all.

I don't think the effect works in all cases. In particular, when a party's position in a nation or region changes along with its size you lose the visual baseline for the comparison and it becomes harder to judge by how much a party's share has changed. Nevertheless, I thought this was an efficient way to summarise a large dataset and the comparison was worth sharing.