Why printing money doesn't necessarily cause inflation

Paul Krugman, most recent Nobel laureate for Economics, shows on his blog a lesson from Japan on how increasing the money supply, even by very large amounts WILL NOT CAUSE INFLATION IN A LIQUIDITY TRAP. The key graph I'll include below.

Whilst during a period of inflation, such a monetary expansion would have made things much worse, in a situation like Japan's since 1990, or the Western world's since last summer, it is all you can do to prevent massive, destructive deflation.