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The Google Charts Tools provide two kinds of heat map charts for geographical data, the Flash based Geomap and the HTML5/SVG based Geochart.

I prefer the Geochart as it doesn't require Flash, but so far there have been two shortcomings with it: I couldn't add additional tooltip information and the default Mercator projection shows Greenland the size of Africa. Both of those issues seemed to have been resolved by Google. Although the features aren't officially documented and released yet, Mitchell Foley from the Google Chart Tools team presented the new developments at the Google I/O 2013 conference in May already.

With version 0.4.3 of googleVis, and thanks to John Muschelli, gvisGeoChart gained a new argument hovervar allowing users to add further information to the tooltip. Additionally, following the examples in Mitchell's presentation I can change the projection as well. The official release from Google shouldn't be too far away.

So, here are again the heat maps of countries' credit ratings from three American and one Chinese rating agency, sourced from Wikipedia. However, this time I use gvisGeoChart, setting the projection to Kavrayskiy VII and the tooltip to the actual rating letter(s), see the R code below.