BFTV, great visualization. May I make a few suggestions:
* As headline numbers have been "fake stable" since 2007, I suggest to do another version, of 2007-2020 only.
* As monthly averages often hide interesting details, I suggest to do it with daily data.
* As the end of September contains a lot of new thin ice, I suggest to do this with data for the 1st-10th of September only. This will give the probability of an ice cover close to the minimum in recent years.
* So the map would be made up of 14 years x 10 days each, and maximal rating would be 140.
* I still think the highest rank ("never had open water") should be white and all the rest a graded color scale.
* While I'm at it, it would be quite interesting to have the same visualization for other seasonal periods, for example 1-10th of August, of July, June. Each of these will answer a different question but in the same effective method. And it fits with A-Tean's discussion of early open water and its effects.
As a comparison, make another 1-10th Sep map for 1979-1992, before the ice began its serious decline. The two maps side by side will tell the story of the change that took place in the Arctic, and how some regions with perrenial ice cover became seasonally ice free, statistically speaking.
I realize what I wrote above is a load of work. Words are easy... but I appreciate any analysis you can provide along these lines. I have long dreamed of making these myself, but my graphic and netcdf skills are very poor unfortunately.