colors = d3.scaleOrdinal(
Array.from(cool_systems.keys()).concat(Array.from(warm_systems.keys())),
new Array(
...d3.range(cool_systems.size).map(v => COLOR_COOL(v / (cool_systems.size - 1.))),
...d3.range(warm_systems.size).map(v => COLOR_WARM(v / (warm_systems.size - 1.))),
),
);