src (unless otherwise specified):
Economic Research Service, “Import Shares of US Food Consumption Using the Volume Method,”
U.S. Department of Agriculture, 2009. Available at: http://www.ers.usda.gov/media/563776/import_1.xls. [found via archive.org]

2013 src:
http://nationalaglawcenter.org/wp-content/uploads/assets/crs/IF10403.pdf

food expend src:
Economic Research Service
Food Price Environment: Interactive Visualization
Accessed March 26, 2018
https://www.ers.usda.gov/data-products/food-price-outlook/food-price-environment-interactive-visualization/

CC’s note: see import_1.xls in DropBox for chart