AP Photo

Americans are clamoring for a fact-based debate about the budget, but the numbers they're hearing from Washington are terribly confusing. Here's an example: Speaking at a Facebook town hall meeting here on Wednesday, President Obama sometimes talked about saving $4 trillion, at other times $2 trillion, and he varied whether it was over 10 years or 12 years, never mentioning any one year.

A simple chart, like the one nearby, would greatly clarify the debate. It shows total federal government spending year-by-year for the two decades starting in the year 2000. Spending is shown as a percentage of GDP, which is a sensible and quite common way to assess trends: When the percentage rises, government spending rises relative to total income or total goods and services produced in our...