Flat for decades, but declining since 2007. The headline is that this explains Trump and Sanders. It's a general problem for someone like Clinton, who is running as the establishment candidate. That's a hard sell right now, even if you didn't have her high personal negatives.
What's more difficult to explain is the delta between President Obama's personal approval ratings, and the right track / wrong track polling. If more than sixty percent of Americans regularly think the country is heading in the wrong direction -- currently over two-thirds -- how is the person normally credited with the greatest personal responsibility for the direction of the nation still about 50/50? George W. Bush's low was 25%, which closely tracked the 23% low for the "right track" figure toward the end of his presidency. You'd expect Barack Obama to be in the same territory. Why isn't he?
If I were to venture a guess, it would be that people aren't telling the truth about how they feel about his performance. Perhaps many people aren't even telling the truth to themselves.