Hi Mehrab,
Have you tried this with BIRT 2.3.2 or 2.5 now?
I can see by looking at the designs that the binding between the data set and the chart is getting messed up for only a single value. Have you experimented with changing the name of that field... like assigning an alias in the query... Just a thought since it only messes up one field. Otherwise, it sounds like a bug to me and I would report it at
Business Intelligence Software - Reporting Tools - BIRT Exchange