Hi Vinod,
Since Category field is part of the table/report you wouldn't see the desired output; Please remove the [Category] field from your table and you will get the output you excpected.
If you want to keep the [Category] then you can read my swuggestion here;
In my previous comment i made a little modification to the solution suggested in this article in odered to get an easier solution.
I put everything here to avoid confussion with other good solutions given above;
[VAR Max Category] = Max([Category]) In ([Lines])
[VAR Concat Category] = If IsNull(Previous(Self;([Lines])))Then [Category] Else [Category]+", "+Previous(Self;([Lines]))
This will prevent concatenating Category fields from other [Lines] and ignor null values hence you don’t need the trick of [VAR Category] as suggested in this article; you need to modify [VAR MaxConcat Category] as follows;
[VAR MaxConcat Category]= [VAR Concat Category] where ([Category]= [VAR Max Category])
The Qualification of this variable should be ‘detail’ of the [Lines]; in the variable editor window set the qualification of this variable to 'detail' and the associated dimension to [Lines].
The above three steps are pretty much working from me in all situations; another advantage of making the last variable a 'detail' of [Lines] is you can use the variable in any report where [lines] is used. This just a little work around on the solution provided by Jothi and all creadit should be given to the orginal solution.
Thanks,
Hailu