Like any technology, charts also come with peripherals: I'm talking about legends, data labels, grid-lines and so on. These things typically give us the most trouble, especially with complex data sets. The analogy is apt: one may feel inextricably knotted up like bunches of cords and wires.
Interactive graphics is a particularly elegant solution to this problem, and Google Finance has done a fantastic job leading the way. One trick is to show the legend only when the user asks for it. Using bar charts (on the left), Google summarizes neatly the performance of stocks within each industry sector. The bar chart gives a sense of the dispersion which adds to the average returns printed next to them. For example, most sectors gained on average but then about 30% of the individual stocks in most sectors actually declined on that day. So the fact that technology stocks gained 0.48% on average doesn't necessarily mean that the two tech stocks you own gained 0.48% or gained at all.
Typically, we would put a legend on the side or at the bottom of the chart, which all be told, is an ugly duckling next to a well-executed chart. Here, the legend is hidden behind the "What's this?" link. The side benefit is that the legend can be as verbose as needed since it doesn't interfere with the chart.
There are a few minor things to consider:
- "What's this?" is not very informative: Why not call it a "legend" or "key"?
- The graph designer seems to think that the most important information sought by readers was the extremes, i.e. the percentage of stocks that gained/lost more than 2%. By darkening the sides of the bar, it draws attention away from the middle which is the boundary between the gainers and the losers. I'd like to see that boundary delineated.
- Similar to the above point, I'd sketch out a version which aligns the gainer/loser boundary to the middle so it's easy to see the balance between gainers and losers. This version however would require more space
- I'd provide sorting by average return, and by percentage of gainers