Seems like their data is a bit odd. I'm under the assumption that they simply curated well known cities.

They didn't have my hometown (Rochester, NY) stating that it is not prone to 90F days. I looked up the weather reports for the last 3 months and found there have been 15 days of >90F temperatures. Meanwhile, Boston had one third the number of 90F days and is included in their data. Nearby cities like Buffalo and Syracuse are included.

It's interesting that the number of hot days hasn't changed. In fact, by the graph, the average has actually declined slightly since my birth year, but rounds to the same whole number. If I make up my birth as being in 1920, the earliest available year, the number of hot days at that time is equal to the number of hot days that are predicted in the future.

I was expecting a completely different result.

I searched Boston and the number of hot days went from 5 in 1987 to 4 in 2018. Although this year we have had about 15 days > 90 degrees. Not sure what to think.

I was born in Mitchell, South Dakota in 1965.

According to the site:

1965: 31 days over 90

2018: 25 days over 90

Interesting.