The Map report type is available in addition to the Trended and Ranked report types. The Map view color codes countries according to their relative contribution to your highest-value line item.
The legend, visible to the right of the Map view, specifies ranges which are calculated in increments of 5th's of your top line item (first 1/5th, second 1/5th, etc). The darker the color, the greater the number of daily unique visitors from that country that are visiting your site.
Only one metric is used when you are in the Map view (more are available in the ranked/trended view).