colour = {
const values = [...dataMap.values()].sort((a, b) => a - b);
const length = values.length >= 6 ? 6 : values.length;
const breaks = simple.ckmeans(values, length).map((d) => d.pop());
return d3
.scaleThreshold()
.domain(breaks)
.range(colourScheme[length + 1].slice(-length));
}