A lovely example of how to lie with statistics. That graph, here's the link again;

https://fullfact.org/health/spending-english-nhs/

does not show spending on the NHS. It shows INCREASE of spending on the NHS.

Decrease in spending is when the graph goes below the base line. It shows three very short instances when this happens. Between 1950 and 1960, the late 1970s and mid to late 1990s. The last one was definitely a Labour Government. The second one is difficult to tell. Government went between Harold Wilson for Labour and Ted Heath for Conservatives and the graph isn't accurate enough to determine who was in power at the time of that dip.