The first place I looked up was England - the Met Office graph seems to indicate a rise. [1]
Have you got any examples of flat graphs?
Miami Florida December 2000 (peak day-average is 77.08): https://www.wunderground.com/history/monthly/us/fl/miami/KMI...
Miami Florida December 2020 (peak day-average is 76.69): https://www.wunderground.com/history/monthly/us/fl/miami/KMI...
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Honolulu, Hawaii December 1995 (peak day-average is 79.92): https://www.wunderground.com/history/monthly/us/hi/honolulu/...
Honolulu, Hawaii December 2020 (peak day-average is 79.54): https://www.wunderground.com/history/monthly/us/hi/honolulu/...
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You can use that site to look up the same month 20+ years apart in any place and it clearly shows that there is no significant change at all. You can browse the surrounding years to get a better feeling because there can be random 2 to 3 degree fluctuations but my point is that these fluctuations can go either way and there is no correlation with time.
TBH, I'm surprised that, given that there is an incentive for meteorologists to fake the data (since global warming is their bread and butter) that it's still possible to see proper data which certainly matches my personal observations. IMO it's a matter of time before they start faking historical data. Good we have waybackmachine https://archive.org/web/
I genuinely cannot tell if this is ill-intent or an internal struggle to revisit your previously held opinions when faced with the body of evidence of all the data collected after the industrial revolution.
You introduce bias by your selection of measurements. Plus a large portion of the earth is scantly inhabited with no weather stations in place. Like the poles or international waters.
Instead you should look at satellite measurements of the temperature across the entire planet. These are widely available. The scientific discourse and the available evidence is much more advanced than "hey it became colder in those 20 places".
https://en.wikipedia.org/wiki/Satellite_temperature_measurem...
Why should I trust a heavily processed, combined result like the Wikipedia page you posted more than random sampling of raw data directly from local meteorologists? It's pretty obvious that the more 'processing' of the data is carried out, the more inaccurate it's going to be. By the time we get to the global mashup of the data, it's been so heavily tempered by people along the hierarchy that it's basically useless. The global statistics are not to be trusted, they're the result of many meteorologists and statisticians playing 'Chinese whispers' at a global scale.
For the Miami case, it is probably worth noting that Miami is cooler during El Nino events and warmer during La Nina. Of course, 2000 was in the middle of the latter and 2020 the former. In fact, if you look at more than 2 datapoints the trend over the past twenty years is the opposite of what you're saying.
Now try doing that systematically. Take the monthly averages over 20 years from a few thousand sites. Don't forget to correct for the urban heat-island effect, which creates a false impression of warming when a city grows out around an airport.
Do that, present your results, and it might be worth taking you seriously.
Picking 2 dates at random is not a good way to expose long-term trends, as evidenced by your observation that there can be several degrees of variation from one year to the next.