group function in view not working

Hi,
I tried created a view associated with a 'expense form', which records daily expense with 'category' field, such as oil, meals...etc. In that view, I select 'category' as grouping field. However, the pie chart still treats every expenses as individual records, not group by any category.
You can access my view here
http://creator.zoho.com/machuang/accountbook/#
and check the expense details and the report I created in the end of list.

Thank you.

Ansel