This bug is being tracked against the Charting Plugin here JCHART-361

When using the Workload Pie Chart gadget, if there's no original estimates in the selected issues JIRA will return a JSON response correctly but with a 503 error code. According to RFC 2616:

503 Service Unavailable
The server is currently unable to handle the request due to a temporary overloading or maintenance of the server. The implication is that this is a temporary condition which will be alleviated after some delay. If known, the length of the delay MAY be indicated in a Retry-After header. If no Retry-After is given, the client SHOULD handle the response as it would for a 500 response.

Note: The existence of the 503 status code does not imply that a server must use it when becoming overloaded. Some servers may wish to simply refuse the connection.

The server is not overloaded or down for maintenance, so 503 is probably not the most appropriate response code when there's simply no issues to display. This can cause issues where the JSON error message isn't displayed when a reverse proxy is being used, for example WebSEAL. WebSEAL rewrites the responses according to the error code returned, so the user gets a big scary "Service unavailable!"-type error message instead of the nice one we're returning.