It's a fun project, but I wish the years weren't locked to 5 year intervals.
EDIT: Actually many years are missing, it seems. For Norway there's nothing between 2000 and 1985, but I guess that's how the charts are pulled?
I also noticed that when you only have these 5 year jumps, certain genres are completely missing. Take US charts - Grunge is non-existent, as it had not yet hit the charts in 1990, and by 1995 it was over.
You are 100% right. I started with the 5 year interval for simplicity, and then I noticed what you pointed out for the exact same reason (I grew up with Grunge so this was very obvious to me as well). I started adding more granularity, so e.g. USA now has the full 2010–2025 charts set. My hope is to populate the whole map with higher temporal density moving forward (the https://88mph.fm/suggest feature is there for that reason too). Will probably have to think of a slightly different UI for the time selector, which is a fun challenge