Brushable parallel coordinates

// Specify the chart’s dimensions.
const width = 928;
const height = keys.length * 120;
const marginTop = 20;
const marginRight = 10;
const marginBottom = 20;
const marginLeft = 10;

// Expose the current selection as a mutable.
const selection = Mutable([]);

// Create an horizontal (x) scale for each key.
const x = new Map(Array.from(keys, (k) => [k, d3.scaleLinear(d3.extent(data, (d) => d[k]), [marginLeft, width - marginRight])]));

// Create the vertical (y) scale.
const y = d3.scalePoint(keys, [marginTop, height - marginBottom]);

// Create the color scale.
const color = d3.scaleSequential(x.get(keyz).domain(), (t) => d3.interpolateBrBG(1 - t));

// Create the SVG container.
const svg = d3.create("svg")
    .attr("viewBox", [0, 0, width, height])
    .attr("width", width)
    .attr("height", height)
    .attr("style", "max-width: 100%; height: auto;");

// Append the lines.
const line = d3.line()
  .defined(([, value]) => value != null)
  .x(([key, value]) => x.get(key)(value))
  .y(([key]) => y(key));

const path = svg.append("g")
    .attr("fill", "none")
    .attr("stroke-width", 1.5)
    .attr("stroke-opacity", 0.4)
  .selectAll("path")
  .data(data.slice().sort((a, b) => d3.ascending(a[keyz], b[keyz])))
  .join("path")
    .attr("stroke", (d) => color(d[keyz]))
    .attr("d", (d) => line(d3.cross(keys, [d], (key, d) => [key, d[key]])))
  .call((path) => path.append("title").text((d) => d.name));

// Append the axis for each key.
const axes = svg.append("g")
  .selectAll("g")
  .data(keys)
  .join("g")
    .attr("transform", (d) => `translate(0,${y(d)})`)
    .each(function(d) { d3.select(this).call(d3.axisBottom(x.get(d))); })
    .call((g) => g.append("text")
      .attr("x", marginLeft)
      .attr("y", -6)
      .attr("text-anchor", "start")
      .attr("fill", "currentColor")
      .text((d) => d))
    .call((g) => g.selectAll("text")
      .clone(true).lower()
      .attr("fill", "none")
      .attr("stroke-width", 5)
      .attr("stroke-linejoin", "round")
      .attr("stroke", "white"));

// Create the brush behavior.
const deselectedColor = "#ddd";
const brushHeight = 50;
const brush = d3.brushX()
    .extent([[marginLeft, -(brushHeight / 2)], [width - marginRight, brushHeight / 2]])
    .on("start brush end", brushed);

axes.call(brush);

const selections = new Map();

function brushed({selection: s}, key) {
  if (s === null) selections.delete(key);
  else selections.set(key, s.map(x.get(key).invert));
  const selected = [];
  path.each(function(d) {
    const active = Array.from(selections).every(([key, [min, max]]) => d[key] >= min && d[key] <= max);
    d3.select(this).style("stroke", active ? color(d[keyz]) : deselectedColor);
    if (active) {
      d3.select(this).raise();
      selected.push(d);
    }
  });
  selection.value = selected;
}

const data = FileAttachment("data/cars.csv").csv({typed: true}).then(display);
const keys = data.then((data) => data.columns.slice(1, -1)); // drop ordinal dimensions