The pie chart shows the activity of your spending for your categories, which span all accounts.
So, when you view a report (pie chart) you are not looking at a pie chart for 1 account, but for all of your spending for all accounts.
At the moment, we do not have account specific charts, but rather are helping you look at all of your spending.
When you only see spending for 1 account it can be misleading if you have major spending in other accounts, so we decided to have the reports show all account data.
If you have a need to see data for only a specific account, we can add it to our future feature list, and it will be added once we prioritize that feature.
Let me know.