Heatmap Visualizations: the NY Times and NASDAQ

The New York Times has published another excellent visualization, this one a heatmap, Can a President Tame the Business Cycle? The on-line, interactive version adds highly useful capabilities — a detail window that pops-up as a mouse-over effect, and alternative, bar-chart visualizations of each of the seven data series — that obviously can't be delivered in a static, printed newspaper. I've noted that paper/electronic gap before. My purpose this go-around is to explore, via comparison with a financial-information visualization published by the NASDAQ stock market, how underwhelming a mediocre heatmap can be.Check out the NASDAQ-100 Dynamic Heatmap, which displays the day's price change for each of the hundred companies that belong to the NASDAQ-100 Index, one company per heatmap cell. On the plus side, the display has a nice day's-summary pop-up for each cell and click-through from each cell to a set of detailed InfoQuotes pages.

The minuses, however, start before you even look at the heatmap. The description of the graphic is incorrect, missing the word change, which I'll insert here: "View the price change of companies in the NASDAQ-100 Index at a glance." But that's just text. The biggest defect is NASDAQ's decision to use a heatmap where a color coded bar chart would deliver more, more usable, information.

How's that?

Heatmaps are good for rendering data values arrayed in a two-dimensional matrix. You have a data table with an independent variable in each of two axes, horizontal and vertical. Cell color conveys magnitude or intensity of a third, dependent variable.

When dimension variables have a lot of values, the heatmap will look like a continuous 2-D surface rather than an array of discrete cells. But if you do have cells rather than point values, you can annotate each with the value of a fourth, dependent variable that is classified, as the third variable is, by the horizontal- and vertical-axis dimensions.

NASDAQ wasted a spatial dimension, the Y (vertical) axis of the data array. That is, the columns in the heatmap have no meaning. Essentially, the X (horizontal) axis is chopped into ten, stacked segments. Further, NASDAQ chose to redundantly annotate cells with the exact value of the variable that was color-rendered. The annotation could have displayed some other useful value, say (given that the color coding shows price percent change since market opening), a volatility index indicating the width of the day's trading range.

A bar chart display, along the lines of the New York Times's drill-downs, would render the NASDAQ data more effectively. Given 100 companies, a conventional bar chart would be overcrowded, but that problem is solvable. A variant capable of hyperbolic display, of selectively enlarging portions of the overall graphic, is perfect when there are many category-variable values. TableLens from Business Objects could do the job well (although the extent TableLens, which was released quite a few years back, is being maintained is unclear.)

My aim in this article was to shed light on effective and on less-than-appropriate use of heatmaps. I hope it helps.The New York Times has published another excellent visualization, this one a heatmap, Can a President Tame the Business Cycle? The on-line, interactive version adds highly useful capabilities that obviously can't be delivered in a static, printed newspaper. Let's explore, via comparison with a financial-information visualization published by the NASDAQ stock market, how underwhelming a mediocre heatmap can be.