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.
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.