No food ?

[Bushface]

A few turns ago, I clicked on the Food Production graph for the first time ever ( I can't really see the point of it, by the way). It showed no food production by anybody until some five or six turns earlier, then all the graphs shot up. I have just tried it again, and now the graph shows no food production at all by any civ, though I have several cities with over 10 pop'n and growing happily.
This in Warlords 2.08, monarch, normal speed, large pangaea.