Domain and Range
It can be confusing at first to remember which values domain and range map to. Ryan Legler helped me remember with the pneumonic, d for data, r for rems.
// domain === data
const data = [3000, 4560, 8340, 123];
// domain is [123, 8340]
// range === rems (it's really pixels
// but the notion of space is true)
const width = 450;
// range is [0, 450]
Scales
You can pass a pixel value to get the domain value by using scale.invert().
const scale = d3.scaleLinear().domain([3000, 4560, 8340, 123]).range([0, 450]);
scale.invert(225);
// 3780