It seems like it may be related to newer versions of Flash, but rolling back to older versions has not restored the functionality. I've tried hosting the Flash component as part of our deployment using the CHART_URL parameter, but with no luck. This reproduces across a variety of browsers and operating systems.
I've been unable to do any real debugging of the problem, because the Yahoo chart component that the API is using underneath is pretty opaque.
Has anyone else seen similar behavior? Any suggestions on how to debug the problem?