Sums sorted by category: Should include also income. Example: Return of goods is an income, which should neutralize the according expense.
Sashi G commented
I agree. It is ridiculous to see my rent amount as two times what I pay because this disregards my flatmate paying me back.
I totally agree...
There should NOT be separate income and expense categories.
Even though some categories could be characterized as income or expense by default, e.g. a salary is by default an income category, most of all other categories can be both "income" AND "expense" at the same time.
For example, "Clothing" can belong to the expense category, as we usually buy clothes, but at the same time when you return something and receive a refund, this should actually credit the "Clothing" category so in this case the "Clothing" category should be considered in the income group.