I'm considering using Google Chart API. Is the API stable? Is the service uptime any good? There are no warranties about it on the Terms of Service. Has anyone known of large downtime periods?
I have been only using it for a couple months, but I haven't had any downtime, or any degradation in chart generation speed.
I doubt Google would let this one go down for long... They use it themselves on Google Analytics. For that fact alone, I feel comfortable using it. Of course, if you want any guarantees, you will need a different solution.