Urgent! Report Server Bind Dataset

Hi Everyone,

I have used the StoredProcedure as shown below:

Begin

Select a.Field1,b.Field from table a, table b where a.patientid=@patID
if(@select=1)

select a.Field1, a.Field2 from table a

else if(@select=2)
select b.Field1, b.Field2 from table b
end

When I bind this storedprocedure with Dataset, all the Fields Corresponding to table a and table b are displayed in the Dataset.Finally, when i bind this Dataset with my chart, the fields corresponding to table b are not displayed in the Chart.
The reason for that may be because Report Server cannot bind with multiple Datasets? Please Clarify.
If the Report Server cannot bind with multiple datasets, then how to call different StoredProcedures (according to Criteria) in a Single Report in Report Server project?

Thanks
Navdeep