Via Alberto Cairo (whose new book How Charts Lie can be pre-ordered!), I found the Water Stress data visualization by the Washington Post. (link)
The main interest here is how they visualized the different levels of water stress across the U.S. Water stress is some metric defined by the Water Resources Institute that, to my mind, measures the demand versus supply of water. The higher the water stress, the higher the risk of experiencing droughts.
There are two ways in which the water stress data are shown: the first is a map, and the second is a bubble plot.
This project provides a great setting to compare and contrast these chart forms.
How Data are Coded
In a map, the data are usually coded as colors. Sometimes, additional details can be coded as shades, or moire patterns within the colors. But the map form locks down a number of useful dimensions - including x and y location, size and shape. The outline map reserves all these dimensions, rendering them unavailable to encode data.
By contrast, the bubble plot admits a good number of dimensions. The key ones are the x- and y- location. Then, you can also encode data in the size of the dots, the shape, and the color of the dots.
In our map example, the colors encode the water stress level, and a moire pattern encodes "arid areas". For the scatter plot, x = daily water use, y = water stress level, grouped by magnitude, color = water stress level, size = population. (Shape is constant.)
The map is far superior in displaying spatial correlation. It's visually obvious that the southwestern states experience higher stress levels.
This spatial knowledge is relinquished when using a bubble plot. The designer relies on the knowledge of the U.S. map in the head of the readers. It is possible to code this into one of the available dimensions, e.g. one could make x = U.S. regions, but another variable is sacrificed.
Non-contiguous Spatial Patterns
When spatial patterns are contiguous, the map functions well. Sometimes, spatial patterns are disjoint. In that case, the bubble plot, which de-emphasizes the physcial locations, can be superior. In our example, the vertical axis divides the states into five groups based on their water stress levels. Try figuring out which states are "medium to high" water stress from the map, and you'll see the difference.
The map handles finer geographical units like counties and precincts better. It's completely natural.
In the bubble plot, shifting to finer units causes the number of dots to explode. This clutters up the chart. Besides, while most (we hope) Americans know the 50 states, most of us can't recite counties or precincts. Thus, the designer can't rely on knowledge in our heads. It would be impossible to learn spatial patterns from such a chart.
The key, as always, is to nail down your message, then select the right chart form.