The Google Chart API supports 9 types of charts (and variations within them liked Stacked Bar etc) but except for Line, Bar, Pie, Radar charts and Scatter plots, the rest including Venn diagrams, Maps, Google-o-meters, QR codes are possibly not charts in the strictest sense of the word. The important thing about this API is that it is URL based. This means no installation of any components is required. The settings & data for the charts are provided though querystring fields and values. As the image is generated by Google's servers, you save on processing and bandwidth. This API option may be suitable for simple scenarios. Google allows 250,000 API calls per day.
Geographic heat maps help visualize location-based data by highlighting trends, anomalies and distributions across a geographic area. For instance if there is an epidemic, a heat map can show at a glance regions where it is intense.
When I came across the Map type in Google Chart API, I realized I could generate a Google Analytics-like Heat Map showing the top 10 countries from where my blog readers come from.
Read my article in EggheadCafe to see how easy it is to generate a geographic Heat map using Google Chart API.