I finally got a chance to do something with it, and it is both simple and powerful. If you're in legend mode, your big data array is four pieces of data at a time, not three. The first two pieces are latitude and longitude. The third is the magnitude of the line that will be drawn (divided by 200 to compensate for their multiplying by 200). So what is the fourth value?
Anything you want. When you specify legend mode, the color function you pass in to the globe's constructor gets that fourth value for each point on the globe. That in turn allows you to define the color of the line based on something other than magnitude (the default).
What that really means is that you get an extra dimension in your data. Height is always height, but legend mode allows you to add non-height information based on the color of the line.
Google's initial example — search language by volume — is a great example. The color of the line comes from the dominant language of the area. Looking at their globe, you can immediately figure out where the high search volume comes from (big cities, mainly) just by looking for the tall lines. You can also see which languages dominate search in a given area. English (blue) covers the United States and the United Kingdom. French (light green) covers France and also Quebec. Portugese (dark green) dominates Portugal and Brazil, and also Madeira. Likewise, the Canary Islands are yellow, because they're a part of Spain.
For a new visualization at work I did with the globe (as usual, NDAs make me cautious about giving more exact details), people at my studio wanted to see the height of the line represent the number of events at a given location. But they wanted the color to represent whether the average data from that spot represented a "good" or "bad" user experience. Little red lines would be unfortunate, but might not get flagged as high priority. Big red lines would be a problem, because it would mean that a lot of players were having a bad experience. Fortunately, there weren't any of those, but there were some long yellow lines, which suggests an area where we could improve the player's interaction with our game.
To get that view, I set the globe to legend mode and set the magnitude field of each point to the sample size. Then I set the fourth data point to a very heterogeneous number that represented the user experience. My color function looks at that number and puts it into a bucket. Good returns green, bad returns red, and a middle ground returns yellow.
That means each line on my globe gives three pieces of information: location on the planet, sample size from that location, and quality of user experience from that location. It's easy to spin the globe and look for red hot spots. It's also just a pleasure to play with the visual, even though you can find the worst spots pretty quickly.
One person saw my work and suggested adding a calculation that would make the line more or less red, for instance, depending on how far over the bad threshold it went. Bright red would mean a really bad experience; dim red would be right at the line. I guess that would give us three and a half data points. Before, we knew it was bad. Now, we'll know how bad it is.
Legend mode makes this all possible.