That a lot of voters are angry this year is a truism. What’s less clear as the campaigns head for big states like Ohio and Florida is the source of the anger.

Tom Edsall fingers the “end of net upward mobility” and the “shrinkage of the middle class.” Norman Ornstein points to the Wall Street bailouts after the financial collapse of 2008. And for that matter, Jared Bernstein of the Center on Budget and Policy Priorities stresses the frustration of decades of stagnant to declining real wages for low- and middle-earning workers.

All of which seems about right. However, the Rust Belt geography of the nation’s anger suggests another, perhaps deeper, explanation for the populist rage that has driven Donald Trump and Bernie Sanders victories in primaries from New Hampshire to Michigan.

Take a look at this single, stark chart depicting the 35-year history of U.S. employment in manufacturing industries.