Control outer diameter of donut shape

[wahlatlas]

Not sure if the donut shape is an established cartographic style in that the outer diameter always equals the grid-size and only the inner diameter is controlled by data but this is what the current implementation in gridviz looks like to me and looks convincing when used on its own.

What I wanted to experiment with, is a ring around a pie chart, maybe as a border or drop shadow or to encode some additional variable. That might turn out to be aesthetically displeasing but I'd like to hear your opinion.

[jgaffuri]

This donut shape is an alternative to proportional circle. It is less compact than a circle, but it better fills the cell space. The issue is for low values, for which the donut may become too thin and thus unlegible. The symbol area is proportional to the the statistical value, just like for proportional circles. It is not a popular cartographic style, but there are examples of maps in some books. For example in "cartography" by Kenneth Field.

The donuts may be combined with pie charts indeed, having the pie chart in the center and the donut as an outer ring. You only need to carefully define the donut and pie chart donuts accorindgly, so that they coincide well.
You could also do it the other way around: A proportional circle in the center and pie chart as a outer ring.

I am not sure neither about how it would look like. It depends a lot on the data. Maybe it is too much information and the message will become to cluttered. It is however a good example of style combination !