It's based on data from the Congressional Budget Office and the Center on Budget and Policy Priorities. Its significance is not partisan (who's "to blame" for the deficit) but intellectual. It demonstrates the utter incoherence of being very concerned about a structural federal deficit but ruling out of consideration the policy that was largest single contributor to that deficit, namely the Bush-era tax cuts.
An additional significance of the chart: it identifies policy changes, the things over which Congress and Administration have some control, as opposed to largely external shocks -- like the repercussions of the 9/11 attacks or the deep worldwide recession following the 2008 financial crisis. Those external events make a big difference in the deficit, and they are the major reason why deficits have increased faster in absolute terms during Obama's first two years that during the last two under Bush. (In a recession, tax revenues plunge, and government spending goes up - partly because of automatic programs like unemployment insurance, and partly in a deliberate attempt to keep the recession from getting worse.) If you want, you could even put the spending for wars in Iraq and Afghanistan in this category: those were policy choices, but right or wrong they came in response to an external shock.
The point is that governments can respond to but not control external shocks. That's why we call them "shocks." Governments can control their policies. And the policy that did the most to magnify future deficits is the Bush-era tax cuts. You could argue that the stimulative effect of those cuts is worth it ("deficits don't matter" etc). But you cannot logically argue that we absolutely must reduce deficits, but that we absolutely must also preserve every penny of those tax cuts. Which I believe precisely describes the House Republican position.
After the jump, from a previous "The Chart That Should..." positing, an illustration of the respective roles of external shock and deliberate policy change in creating the deficit.
UPDATE: Many people have written to ask how the impact of the "Bush-era tax cuts," enacted under George W. Bush and extended under Barack Obama (with the help, as you will recall, of huge pressure from Senate Republicans), is divided between the two presidents. I don't know and have written the creators of the chart to ask. (They have responded to say: it indicates the legacy effects of the changes made by each Administration. For instance, neither Bush nor Obama is credited with the entire cost of Pentagon spending or entitlements, but only the changes his Administration made, up or down. By this logic the long-run effect of tax cuts initiated by Bush is assigned to him, as any long-run effect of savings he initiated would be too.)
But to me it doesn't matter. As I said above, the point of the chart really isn't partisan responsibility. It is the central role of those tax cuts in creating the deficit that is now the focus of such political attention. Call them the "Obama-Extended Tax Cuts" if you'd like: either way, a deficit plan that ignores them fails a basic logic, math, and coherence test.
From this item three months ago:
More: For how the Democrats are mishandling both the politics and the substance of this argument, see Joshua Green on "The Democrats Cave."
For how President Obama could use his inherent powers, in a "This is bullshit" way, see Robert Kuttner.