Ridgeline plot

Ridgeline plots are an alternative to horizon charts and small-multiple area charts that allow greater precision for a given vertical space at the expense of occlusion (overlapping areas). Data: Christopher Möller

This form is also known as a joy plot for its use on the cover of Joy Division’s Unknown Pleasures album. This name is controversial as it references the sexual slavery practices of the Nazis during World War II; however, Joy Division sought to draw attention to the danger of fascism to society, not to celebrate it.

// Prepare the series.
const dates = Array.from(d3.group(traffic, (d) => +d.date).keys()).sort(d3.ascending);
const series = d3.groups(traffic, (d) => d.name).map(([name, values]) => {
  const value = new Map(values.map((d) => [+d.date, d.value]));
  return {name, values: dates.map((d) => value.get(d))};
});

// Specify the chart's dimensions.
const overlap = 8;
const width = 960;
const height = series.length * 20;
const marginTop = 40;
const marginRight = 20;
const marginBottom = 30;
const marginLeft = 120;

// Create the scales.
const x = d3.scaleTime()
    .domain(d3.extent(dates))
    .range([marginLeft, width - marginRight]);

const y = d3.scalePoint()
    .domain(series.map((d) => d.name))
    .range([marginTop, height - marginBottom]);

const z = d3.scaleLinear()
    .domain([0, d3.max(series, (d) => d3.max(d.values))]).nice()
    .range([0, -overlap * y.step()]);

// Create the area generator and its top-line generator.
const area = d3.area()
    .curve(d3.curveBasis)
    .defined((d) => !isNaN(d))
    .x((d, i) => x(dates[i]))
    .y0(0)
    .y1((d) => z(d));

const line = area.lineY1();

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

// Append the axes.
svg.append("g")
    .attr("transform", `translate(0,${height - marginBottom})`)
    .call(d3.axisBottom(x)
        .ticks(width / 80)
        .tickSizeOuter(0));

svg.append("g")
    .attr("transform", `translate(${marginLeft},0)`)
    .call(d3.axisLeft(y).tickSize(0).tickPadding(4))
    .call((g) => g.select(".domain").remove());

// Append a layer for each series.
const group = svg.append("g")
  .selectAll("g")
  .data(series)
  .join("g")
    .attr("transform", (d) => `translate(0,${y(d.name) + 1})`);

group.append("path")
    .attr("fill", "#ddd")
    .attr("d", (d) => area(d.values));

group.append("path")
    .attr("fill", "none")
    .attr("stroke", "black")
    .attr("d", (d) => line(d.values));

display(svg.node());

const traffic = FileAttachment("data/germany-traffic.csv").csv({typed: true}).then(display);