Interesting chart, but co-relation is not causality. Many factors influence criminal behavior, and the prospect of getting killed by a home-owner certainly will make criminals more unwilling to break into homes. Personally I expect that incarcerating more career criminals (e.g. 3 strikes legislation) has more to do with the drop in crime, given that a relatively small number of offenders commit the majority of crimes. Take them out of circulation for long periods of time and the crime rate will certainly fall. That said, it is my opinion that the 2nd Amendment does protect an individual right, and since the 14th Amendment, the states are limited as well. The legal question (imho) is what standard - rational, intermediate, or strict scrutiny should be applied to measure the governmental interests opposing the individual interests. I think the framers spell it out clearly. By definition the militia referred to in the 2nd Amendment has the missions defined in Article 1 Section 8 of the Constitution ("To provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions;") then arms which are useful in carrying out these missions are clearly protected as an individual right. A useful shorthand for this (as per the Miller decision) is "whatever arm is used by the army."
You and I shared our first impression, I think. The chart does not show that an increase in ownership reduced crime. However, it does negate the liberal argument that an increase in ownership increases crime.