SSRS expression based on DataSet2

when using expression y value in graph in ssrs, can choose "fields (dataset1)" category, can ex: avg(fields!amount.value). 

but how choose fields data set (dataset2)?  formula avg(fields!amount.value, "dataset2") no good, returns single value average entire dataset.

thanks.

the data must have some relationship, example time.  if creating series department, join 2 on time.  common scenario , require full outer join, like

select * (

select department, month, amount sometable department = 'sales') salestb

full outer join

(select department, month, amount sometable department = 'accounting') accountingtb

on salestb.month = accountingtb.month

patrick's point of joining data in source query best practice , efficient way go addressing problem.

hope makes more sense,

mark

 

SQL Server  >  SQL Server Reporting Services, Power View