For those here not scientifically learned...
There is something called ‘significant figures’ used when you take measeurements. The reason being that you cannot be more precise than your measurement instruments.
If all you have is a yard stick, you can never measure something to millimeter precision.
If you have a thermometer calibrated to degrees, you can never be more precise than a half or quarter of a degree or so.
If all you have is tree rings you CANNOT say the temperature in degrees, EVER.
You can make general assumptions that one year was warmer than the previous by the width of the rings, but you can never know the precise temperature was 57 degrees on any year, if you never had a thermometer to measure the degrees. Especally temperatures 2000 years ago.
This is why global warming and the scientists who promote it are frauds
50% of all “science” is bunk.
I’m a little suspicious of this chart for the reasons you describe. And as a certified dumb guy, even I find it hard to believe that there is some kind of nexus between global cooling and volcanic activity. Any thoughts?