His point isn't that the public sector should shed a proportional number of employees, but that the collective income of private sector taxpayers has decreased, yet they are forced to sustain public-sector pensions with that decreased income.
It's hard for me to come up with a good reason for that particular choice of graph other than "political spin". One of many reasons I've switched to reading The Economist rather than the Wall Street Journal for economics-meets-politics news, since the Economist makes a more conscientious effort imo to avoid slanting their data or infographics in the direction of their editorial line (and they do take roughly the same free-market editorial line).