I searched for the lowest annual temperatures over the last 60 years, and I made this graph showing Cincinnati, OH's climate transformation from zone 6A to 7A. I'd be curious if this type of historical weather graph already exists somewhere. Also, I wonder how your climates have changed across the United States over the last 60+ years.

 

Looks like the polar vortex nearly set things straight though eh?

According to this USDA website Cincinnati is zone 6b but nevertheless the trend is clear.

https://planthardiness.ars.usda.gov/phzmweb/Default.aspx#

I am guessing that there is a similar trend where I live, the two polar vortex winters notwithstanding.  This has been a hotter than normal summer, for what its worth.

 
 
 
As you see, Cincinnati, OH, has just been changed to zone 7A, according to The Arbor Day Foundation. It is the border of 6B, 7A, but you see the trend.

Interesting, are those low temps the lowest for each given year? What were the highs? The charts should show the same pattern.

Wow, there definitely looks like a warming trend last few years.  I hope the trend doesn't reverse and start getting cold again like it has in  past cycles.
 
Classic example would be the mini ice age of the 16 and 17th centuries which followed the mediaeval warm period of 1000-1300.

A 50 year temperature graph is a such a small window in time.

Dennis, my original post was a graph of the absolute lowest temp for each year. I didn't look at the highs, but it would be interesting to find out how that changed over time.

60 years is a drop in the bucket of this planet's weather history, but nonetheless, I find tracking the change to be interesting. Hopefully we can stay in zone 7, so I can plant many more fig trees in the ground!