Taxonomy of Data Visualization Methods - Data Visualization: A Successful Design Process - page 147

Description : This type of mapping plots differently-sized circular markers over
given geographical coordinates to indicate the magnitude of a quantitative value.
Whereas the dot plot maps were like geographical scatter plots, these are essentially
bubble charts overlayed on to a map. The main contention with these designs tend
to be that the spread of bubbles, depending on their size, can reach far beyond their
geographical point and end up bleeding into other circles. Normally, the colors used
include a relatively high transparency setting in order to accommodate the potential
overlaps and "halos" are often used to distinguish outer edges. An example of a
bubble plot map is shown in the following screenshot:
Image from "Visualizing Yahoo! Mail" ( http://www.periscopic.com/#/work/yahoo-mail/ ),
created by Periscopic
Isarithmic map (or contour map or topological map)
Data variables : Multiple x quantitative, multiple x categorical.
Visual variables : Position, color-hue, color-saturation, color-darkness.