What does “the line turns on somebody’” mean?

I came across the phrase ‘the line had turn on us’ in the following paragraph of the article titled “Running in the red: How the U.S. on the road to surplus, detoured to massive debt,” appearing in Washington Post (April 30.)

“Still, Hoagland (CBO analyst, William Hoagland), said, the abandonment of fiscal discipline in the wake of the surpluses clearly didn’t help. ‘Nobody pushed for paying for this stuff,’ he said. Not even after “it became very clear in the middle of 2003 that the line had turned on us. And the surpluses as far as the eye could see were no longer there.’”

I searched for the definition of ‘the line had turn on somebody,’ through Google to find no entry of this phrase. Isn’t “the line turns on somebody’” an idiom? What does ‘the line’ here stand for, and what does this phrase mean at all?

He's talking about the line as a plotted line on a graph. At one point the U.S. had been putting up surpluses, but expenditures rose to cancel those out and put the country into a deficit again. The "line" turned and went below even — right at around the point where Bush passed his tax cuts and engaged us in two costly wars.

Here's the line he's talking about:

And you can read more about it.

This is an example of a more general idiom. To say that something has happened "on" someone adds (to the bare statement that it has happened) the implication that the fact that it has happened has affected him adversely. Some examples would be: 1) My car broke down on me. 2) The weather turned bad on them. 3) The winds shifted on him. 4) Our business venture failed on us. I don't think this is related to the idiom "to turn on" someone, which means to betray, as noted.