Passing parameters in a Chart

I have a main report that has one parameter : GroupNameWhen I pass the Group name it pulls up a Bar char based on the data set query :SELECT TOP 10 Category ,SubCategory ,count(ID) as No_Of_People FROM [dbo].[EmpCount] Owhere GroupName = @Group It works fine till here.Now I need to see the names of people when I drill down into a webpage. This will need 3 parameters category and Subcategory and GroupName. I am only passing group as parameter for the main report. How can I pass the category and subcategory from the chart ? . I tried creating an internal parameter , but when I try to get the available value from the dataset , It gives me an error " Parameter validation failed".Any Ideas ???