The Google Chart API returns a PNG-format image in response to a URL. Several types of image can be generated: line, bar, and pie charts for example. For each image type you can specify attributes such as size, colors, and labels.
Check out my fancy Negative Approach (I am 50/50 today) that I made simply by manipulating the URL string.
As described on InfoQ, Deepak Jois has written a wrapper for the API called gchartrb, which provides a clean, concise way to generate chart URLs using Ruby.