Later Tuesday, President Obama will release his annual budget request -- an important statement outlining his plans for the nation and the economy in his final three years of office. But presidents often don't get what they want. The eventual total can differ from those in the president's budget for one of two reasons. One is opposition from Congress, which might push for higher or lower spending. The second is that emergencies, such as, say, the Korean War or Hurricane Katrina can push up expenditures beyond what anyone could have predicted.

The first chart in the below graphic shows the percent difference between the presidential requests and the spending enacted in a given year since 1945. The second chart shows the total amount of taxes collected in each given year. And the third chart shows the total level of discretionary spending in each given year.