Good online visualization\charting API is a great time saver when building a site. It spares the need to install libraries, handle with image files, and write lots of boilerplate code.
Criteria for good answers:
- Good looking
- Stable API and Site (unlikely to disappear anytime soon, leaving my site without charts)
- Good SLA
- Easy to learn
One tool per answer please, and images are really welcome!
PS Admins, please retag my question with "chart" and "visualization"