I'm not sure with this implication, since it's the ratio that would matter? If you switch the chart to a y log scale, the trajectories look parallel

Suggestion to add a note that we express years as a UTC date of 1st January of each year? Not sure what would be the consequence if years were numbers, besides having commas in the formatted ticks.

i debated this too, if we use numbers then doing naive grouping will be easier (dont need InternMap via d3 group). but plot formats the dates nicer by default this way too