I was the lead designer of the market map display (for SmartMoney, which licensed it to the cited site) and am also a Tufte admirer, so this thread is fascinating to me. Here are a couple of comments.
All kinds of experts do indeed use this device to analyze data, but not necessarily in an unhurried, methodical style.
Let me describe one expert I observed. Her job is to execute large trades for institutional investors. She can't simply announce she wants to buy $1,000,000 of Ford, since the price would instantly skyrocket. So instead she arranges many small carefully timed trades to keep prices steady. (Her activities are nothing like classic long-term investing but she is serving a critical social goal by maintaining market liquidity.)
To time her trades she must continually make guesses about near-term market activity, based on data from three computer screens, a TV, and a large overhead board. She told me the SmartMoney map gives her a sense of the overall "market weather" far faster than a table-based display, and she generally devotes one of her three screens to the map.
Her case is typical of expert users of the map. They are usually in a hurry and must absorb data from multiple sources. They often keep the map in view all day and monitor it with frequent glances, occasionally "drilling down" into it with the mouse for more data.
Given that context, I can describe why we made some of the design decisions we did.
First, the goal of using area to represent market capitalization is to direct attention and give an effective macro/micro reading. Bigger companies and sectors get more visual emphasis than smaller ones.
The macro/micro reading comes from combining colors and areas: e.g. if you look at the map across the room you get an instant sense of a key stock market summary statistic, the overall market-cap weighted performance. This kind of "glanceability" and attention management is critical for viewers in a hurry. On the other hand, precise market cap comparisons--problematic due to the area encoding--are generally not important to traders.
Second, the original version of the map had more text, including a ticker symbol inside each company's rectangle. But the more we used and tested the map, the more text we removed. We discovered the company labels interfered with a quick reading of the display. And, surprisingly to us, test users rapidly learned which rectangles corresponded to which sectors and industries, and even the location of many of the companies--for habitual users, the display becomes like a geographic map of a familiar region where labels are redundant. Compare the maps on pp. 17-19 of VDQI or pp. 40-41 of Envisioning Information. Even so, given the needs of first-time users I'm still not sure we made the right decision, and there may well be a way to put more text on the screen without hurting the overall legibility.
Finally, to compare with a well-designed table, I recommend looking at page C2 of the Wall Street Journal. The data is not quite the same, but the goal--providing an overview of market activity--is parallel. I think the table and map are each useful but vastly different in what they convey best. To compare, look at both the table and the map before the market opens on a couple of successive days and see for yourself the difference in perspective.
-- Martin Wattenberg (email)