java.util.collection paramter from main to Sub report

Hi,

I've chart report which uses the parameter of Type Java.util.collection. When I try to use this chart  at group band as subreport to the Main report with similar parameter of Type Java.util.collection, the chart value keep iterates within the group. For example, if the chart shows the value for L1 & L2 in two separate page. But when added to the Main report for the value L1, the subreport displays both L1 & L2 rather than just L1. 

Have anyone worked with passing collection parameter from main to Sub report? Please advise

FYI, using Jaspersoft Studio 5.6.1

Thanks

Sreeram