The UK line is particularly interesting, since that is the country that Krugman has declared is austerity-izing itself into a depression. As I have pointed out before, real government spending in UK has been and is still rising. The percent of GDP of this spending has fallen a bit, but there is nothing about Keynesian stimulus theory that says changes in the percentage of government spending is stimulative, only its absolute value.
Here is one thing I would love to here Krugman et. al. opine on -- at what percentage of government debt to GDP does additional deficit spending become counter-stimulative. I imagine there is an inverse relationship for deficit-funded stimulus, such that it has a larger effect at lower debt levels with a zero to negative effect at higher interest levels.
Update: From another source, here is the UK in real $