Items in the income section of your report or graph show negative amounts when an income category is used in a payment transaction. Income categories are typically used in deposit transactions to categorize income.
Double-click the item in the report or graph to go directly to that transaction in the register. Check to see whether you've accidentally entered the deposit amount in the payment column. If this transaction should be a payment, check the Category List to make sure that the category that you used is an expense category.