So by now you've certainly seen this chart from Paul Ryan, showing what will happen to the national debt if current policy is left in place vs. what will happen under his roadmap.

Take the "current path" rather than the "Path To Prosperity" certainly seems scary. That choice is Paul Ryan's main thing, although he presents his scary vision in an incredibly sunny and positive manner, which might make him a great campaigner for Mitt Romney.

But since we're talking about budget charts, we thought we'd show you a few that aren't likely to come up on the campaign trial.

The first is Federal Interest Outlays as a percentage of GDP. Basically, it looks at how much it's costing the government to service existing debt.

As you can see, we're very close to historical lows by this measure, and in far better shape than we were during the '80s and '90s.

Of course, a big reason for the decline in interest payments is that interest rates have come down a lot.

The crucial idea here is that contrary to what a lot of people think, interest payments have no reason to go up alongside the size of the national debt.

In fact, historically, the relationship has been the inverse.

So here's the national debt (blue line) vs. the yield on the 10-Year Treasury (red line), which is a good benchmark for the government's cost of borrowing.

Furthermore, this isn't some freak historical accident or a function of the fact that the Fed is trying to depress rates or even having anything to do with the U.S. dollar as a global reserve currency.

The norm is that countries with higher debt loads pay less to borrow.

Here's a chart we made last November comparing debt-to-GDP ratios across various countries and their costs of borrowing.

Greece is a real freak-case. Japan (whose economy the U.S. most resembles on a fiscal, monetary, and demographic basis) is a very clear example of what happens to borrowing costs over time.

Conversely, what people call budget "soundness" can turn out to be very bad.

Bill Clinton famously "balanced the budget" in his second term in office. Soon thereafter, the economy went into a tailspin. The balanced budget provided no fortitude against this decline.

Japan also had a balanced budget in the late '80s right before its decline.

Lest you think that this is some kind of spurious causation/correlation failure, consider that a lack of safe assets in the late '90s (due to declining Treasury issuance) forces investors into riskier areas for return: It's no surprise that at the same time as Clinton's budget was being balanced, we saw a bubble in stocks, and the beginning of the great housing-bubble, as the world clammered for de-facto government backed assets from Fannie and Freddie.

Bottom line: Government budget math isn't nearly so straightforward as Paul Ryan would have you believe, and the size of the debt/deficit is not necessarily a good proxy for economic health.