Why does the sign change for Net Income on each rail?

"Money never starts an idea; it is the idea that starts the money." -W. J. Cameron

The top rail is the income rail. It tells you how much net income you have achieved for a particular period by subtracting all your expenses from you income.
The bottom rail is a cash flow rail and is measured from a debt perspective.
i.e. will the amounts ADD to debt or SUBTRACT from debt.
When you make Net Income it will subtract from debt which is why the sign changes when the figures move onto the bottom rail.

"Twenty years from now you will be more disappointed by the things you didn't do than by the ones you did do. So throw off the bowlines. Sail away from the safe harbor. Catch the trade winds in your sails. Explore. Dream. Discover." - Mark Twain

http://www.wheresthemoneygone.com/demo.html