In particular, "number of firearms per capita" does not equate to firearm ownership percentage because people who own a gun typically own several firearms.
Households with firearms in the USA is roughly 32%.
I'd say the actual numbers are way up, and that self-reported numbers are down since 2008. This graphic shows a decline in ownership overall in both individuals and households from 2008 to 2010. Given what we know and have seen since then, I'd say these graphs are bunk. Especially since they come from the VPC.
Your chart is from an anti-gun propaganda org website.
They claim only 20% of the population own guns. Even liberals would cringe at BS numbers like that.