Our API directory now includes 38 visualization APIs. The newest is the Voicebox API. The most popular, in terms of mashups, is the Google Visualization API. We list 26 Google Visualization mashups. Below you'll find some more stats from the directory, including the entire list of visualization APIs.
Chart APIs are useful and popular, and Google knows how to do them well, so they're continuously adding more features and options for customization to their Visualization API. The Google Code Blog recently highlighted a few of the newest features.