Something to keep in mind when you hear references to part-time work as an economic indicator is that most people who have part-time jobs want part-time jobs. The Bureau of Labor Statistics' survey asks people who say they work part-time why they're working part-time, and classifies a sub-set of them as "part-time for economic reasons." Those are people who say they're looking for a full-time job but can't find one, or that there aren't enough shifts available at their current workplace. What you see above is the ratio of part-time for economic reasons workers to all part-time workers.
As you would expect, the recession led to a large increase in the economic reasons share. But even at the trough of the labor market, something like two-thirds of part-time workers were voluntarily part-time. These are people who are also in school, or people with kids at home, or people who've entered a semi-retirement phase of life.