globalTempsByCountryByYear = d3
.rollups(
globalTempsByCountry,
(v) => d3.mean(v, (e) => e.AverageTemperature),
(d) => d.Country,
(d) => d.date.getFullYear()
)
.map(([Country, temps]) =>
temps.map(([Year, AverageTemperature]) => ({
Country,
Year,
AverageTemperature
}))
).flat()