Bubble charts force three dimensions onto a 2-D flat surface. They are occasionally useful for illustrating concepts but seldom work as a data graphic. The following chart illustrates some lethal problems:
In bubble charts, two dimensions are plotted in the usual x and y axes (here, longitude and lattitude) while the third dimension is depicted as circular areas. Like wild dogs, the pair of gigantic bubbles insisted on marking their territories, obscuring many littler bubbles. At the same time, it gets harder and harder to locate their centers (i.e. recover the other two dimensions) as bubbles expand.
Further, the standard way of displaying the legend, involving overlapping circles, obstructs our ability to compare the areas effectively.
This chart, however, is data-rich. Simultaneously, it plots (1) geographical locations (map); (2) passenger volume (area of circle); (3) market share (shading of circle); and (4) top markets (call-out text). The key to improving readability is to untether oneself from geography. In other words, give up geographical information and focus on market share versus passenger volume. For example:
[The graph looks better if it had only blue diamonds. I had to insert green dots because I don't have the full data set. The blue diamonds are real data; green dots are approximations, and there should have been many more in roughly the same locations.]
We now see that Northwest's markets fall into three types: large and dominant (national hubs), small and strong (regional hubs), and small and small (others). There are only a few cities with market share over 50% while the rest are less than 25%. Similarly, NWA serves fewer than 25,000 passengers in all but three markets. (A log scale can be used here if one wants to explore further groupings within the small-scale markets.)
Reference: "Well-Laid Plan Kept Northwest Flying Despite the Strike", New York Times, August 22, 2005