Radial Area Update Pattern

D3.js's general update pattern used to create a transitioning radial area chart. This chart format can be useful for showing cyclical data, such as how a value changes through the year. For an excellent example, see "The Baby Spike", by the great Nadieh Bremer.

<!DOCTYPE html>
<html>
  <head>
    <style>
      body {
        margin: 0;
        font-family: "Helvetica Neue", sans-serif;
      }
      svg {
        margin: 0 auto;
        display:  table;
      }
      .area {
        fill:  steelblue;
        fill-opacity:  .5;
      }
      .tick circle {
        fill: none;
        stroke:  #888;
      }
      .tick line {
        stroke: #888;
      }
      .tick text {
        font-size: 12px;
        text-anchor: middle;
      }
    </style>
  </head>
  <body>
    <script src="https://d3js.org/d3.v5.min.js"></script>
    <script src="https://d3js.org/d3-scale.v3.min.js"></script>
    <script src="https://unpkg.com/geometric@2.0.1/build/geometric.min.js"></script>
    <script>
      // Dimensions
      const margin = {left: 28, right: 28, top: 28, bottom: 28};
      let width, height;

// Data
      let data = [];
      const days = d3.timeDay.range(new Date(2019, 0, 1), new Date(2020, 0, 1));

// Scales
      const xScale = d3.scaleTime()
          .domain(d3.extent(days))
          .range([0, Math.PI * 2]);

const yScale = d3.scaleRadial()
          .domain([0, 60]);

// Generators
      const areaGenerator = d3.areaRadial()
          .angle(d => xScale(d.date))
          .innerRadius(d => yScale(d.v0))
          .outerRadius(d => yScale(d.v1))
          .curve(d3.curveBasis);

// Elements
      const svg = d3.select("body").append("svg");
      const g = svg.append("g");

const xAxis = g.append("g")
          .attr("class", "axis");

const xAxisTicks = xAxis.selectAll(".tick")
          .data(d3.timeMonth.every(1).range(...d3.extent(days)))
        .enter().append("g")
          .attr("class", "tick");

xAxisTicks.append("text")
          .attr("dy", -15)
          .text(d => `${d3.timeFormat("%b")(d)}.`);

xAxisTicks.append("line")
          .attr("y2", -10);

const yAxis = g.append("g")
          .attr("class", "axis");

const yAxisTicks = yAxis.selectAll(".tick")
          .data(yScale.ticks(4).slice(1))
        .enter().append("g")
          .attr("class", "tick");

const yAxisCircles = yAxisTicks.append("circle");

const yAxisTextTop = yAxisTicks.append("text")
          .attr("dy", -5)
          .text(d => d);
      
      const yAxisTextBottom = yAxisTicks.append("text")
          .attr("dy", 12)
          .text(d => d);

// Updater
      const duration = 750;
      makeData();
      redraw();
      onresize = _ => redraw(true);
      d3.interval(_ => {
        makeData();
        redraw();
      }, duration * 2);

function redraw(resizing){
        const diameter = Math.min(innerWidth, innerHeight);
        width = diameter - margin.left - margin.right;
        height = diameter - margin.top - margin.bottom;

yScale
            .range([0, height / 2]);

svg
            .attr("width", width + margin.left + margin.right)
            .attr("height", height + margin.top + margin.bottom);

g
            .attr("transform", `translate(${margin.left + width / 2}, ${margin.top + height / 2})`);

xAxisTicks
            .attr("transform", (d, i, e) => {
              const point = [width / 2, 0];
              const angle = i / e.length * 360;
              const rotated = geometric.pointRotate(point, 270 + angle);
              return `translate(${rotated}) rotate(${angle})`;
            });

yAxisCircles
            .attr("r", d => yScale(d));

yAxisTextTop.attr("y", d => yScale(d));

yAxisTextBottom.attr("y", d => -yScale(d));

// General update pattern for the area, whose data changes
        const area = g.selectAll(".area")
            .data([data]);

if (resizing){
          area
              .attr("d", areaGenerator);
        }
        else {
          area.transition().duration(duration)
              .attr("d", areaGenerator);
        }

area.enter().append("path")
            .attr("class", "area")
            .attr("d", areaGenerator)
            .style("opacity", 0)
          .transition().duration(duration)
            .style("opacity", 1); 
      }

// Functions for generating random data
      function makeData(){
        let v0 = randBetween(5, 25);
            v1 = randBetween(40, 60);

data = days.map(date => {
          v1 = Math.min(v1 + random([-1, 1]), 60)
          v0 = Math.min(Math.max(v0 + random([-1, 1]), 1), v1 - 5)
          const obj = {
            date,
            v1,
            v0
          };
          return obj;
        });
      }

function randBetween(min, max){
        return Math.floor(Math.random() * (max - min + 1) + min);
      }

function random(arr){
        return arr[randBetween(0, arr.length - 1)];
      }
    </script>
  </body>
</html>

https://d3js.org/d3.v5.min.js

https://d3js.org/d3-scale.v3.min.js

https://unpkg.com/geometric@2.0.1/build/geometric.min.js