Northern hemisphere sea ice extent

line {
            stroke: #E9E9E9;
        }

.coil-label {
            fill: #000;
            font-size: 12px;
        }
        /*.arc path {
            stroke: #FFF;
        }*/
    </style>
</head>

<body>
    <div id="title">
        <h1>Northern hemisphere sea ice extent: 1978 to 2017</h1>
        <p>An example of a resuable spiral heatmap showing the extent of the sea ice in the northern hemisphere.</p>
        <p>Low sea ice is starting earlier in July, and lasting longer into October and November.</p>
    </div>
    <div id="legend"></div>
    <div id="chart"></div>
    <div id="source">
        <p>Source: https://nsidc.org/data/nsidc-0081.html and https://nsidc.org/data/nsidc-0051.html</p>
    </div>

let dateParse = d3.timeParse("%d/%m/%Y")
        let dateFormat = d3.timeFormat("%d%m");// returns date string ddmm, eg 2902
        let dayOfYear = d3.timeFormat("%j");

let startAngle = 0;

let arcLabels = [
            { "month": "Jan", "start": 0, "days": 31 },
            { "month": "Feb", "start": 31, "days": 28 },
            { "month": "Mar", "start": 59, "days": 31 },
            { "month": "Apr", "start": 90, "days": 30 },
            { "month": "May", "start": 120, "days": 31 },
            { "month": "Jun", "start": 151, "days": 30 },
            { "month": "Jul", "start": 181, "days": 31 },
            { "month": "Aug", "start": 212, "days": 31 },
            { "month": "Sep", "start": 243, "days": 30 },
            { "month": "Oct", "start": 273, "days": 31 },
            { "month": "Nov", "start": 304, "days": 30 },
            { "month": "Dec", "start": 334, "days": 31 }
        ]

//Colour scale
        var colour = d3.scaleSequential(d3.interpolateSpectral);

//Load the data, nest, sort and draw charts
        d3.csv("sea-ice-extent.csv", convertTextToNumbers, function (error, data) {
            if (error) { throw error; };

colour.domain(d3.extent(data, function (d) { return d.extent; }));

let chartData = createDataPerDay(data);

startAngle = ((data[0].dayOfYear / 365) * 360) - 1;

//set the options for the sprial heatmap
            let heatmap = spiralHeatmap()
                .radius(chartRadius)
                .holeRadiusProportion(0.1)
                .arcsPerCoil(365)
                .startAngle(startAngle)
                .coilPadding(0.1)
            //.coilLabel("year")

//CREATE SVG AND A G PLACED IN THE CENTRE OF THE SVG
            const div = d3.select("#chart").append("div")

const svg = div.append("svg")
                .attr("width", chartWidth + margin.left + margin.right)
                .attr("height", chartHeight + margin.top + margin.bottom);

const g = svg.append("g")
                .attr("transform", "translate("
                + (margin.left + chartRadius)
                + ","
                + (margin.top + chartRadius) + ")");

g.datum(chartData)
                .call(heatmap);

g.selectAll(".arc").selectAll("path")
                .style("fill", function (d) { return colour(d.extent); })

d3.selectAll(".arc-label").remove();

var arcLabelsG = g.selectAll('.arc-label')
                .data(arcLabels)
                .enter()
                .append('g')
                .attr('class', 'arc-label')

let dayAngle = 360 / 365;

arcLabelsG
                .append('text')
                .text(function (d) {
                    return d.month
                })
                .attr('x', function (d) {
                    let monthAngle = 360 * (d.days / 365);
                    let labelAngle = (d.start * dayAngle) + (monthAngle / 2)
                    let labelRadius = chartRadius + 20;
                    return labelX(labelAngle, labelRadius)
                })
                .attr('y', function (d) {
                    let monthAngle = 360 * (d.days / 365);
                    let labelAngle = (d.start * dayAngle) + (monthAngle / 2)
                    let labelRadius = chartRadius + 20;
                    return labelY(labelAngle, labelRadius)
                })
                .style('text-anchor', function (d, i) {
                    return i < arcLabels.length / 2 ? 'start' : 'end'
                })

arcLabelsG
                .append('line')
                .attr('x2', function (d, i) {
                    let lineAngle = d.start * dayAngle
                    let lineRadius = chartRadius + 40
                    return labelX(lineAngle, lineRadius)
                })
                .attr('y2', function (d, i) {
                    let lineAngle =  d.start * dayAngle
                    let lineRadius = chartRadius + 40
                    return labelY(lineAngle, lineRadius)
                })

//DRAW LEGEND

const legendWidth = chartRadius;
            const legendHeight = 20;
            const legendPadding = 40;

const units = "million square km"

var legendSVG = d3.select("#legend")
                .append("svg")
                .attr("width", legendWidth + legendPadding + legendPadding)
                .attr("height", legendHeight + legendPadding + legendPadding);

var defs = legendSVG.append("defs");

var legendGradient = defs.append("linearGradient")
                .attr("id", "linear-gradient")
                .attr("x1", "0%")
                .attr("y1", "0%")
                .attr("x2", "100%")
                .attr("y2", "0%");

let noOfSamples = 20;
            let dataRange = colour.domain()[1] - colour.domain()[0];
            let stepSize = dataRange / noOfSamples;

for (i = 0; i < noOfSamples; i++) {
                legendGradient.append("stop")
                    .attr("offset", (i / (noOfSamples - 1)))
                    .attr("stop-color", colour(colour.domain()[0] + (i * stepSize)));
            }

var legendG = legendSVG.append("g")
                .attr("class", "legendLinear")
                .attr("transform", "translate(" + legendPadding + "," + legendPadding + ")");

legendG.append("rect")
                .attr("x", 0)
                .attr("y", 0)
                .attr("width", legendWidth)
                .attr("height", legendHeight)
                .style("fill", "url(#linear-gradient)");

legendG.append("text")
                .text("Low (" + colour.domain()[0] + " " + units + ")")
                .attr("x", 0)
                .attr("y", legendHeight - 35)
                .style("font-size", "12px");

legendG.append("text")
                .text("High (" + colour.domain()[1] + " " + units + ")")
                .attr("x", legendWidth)
                .attr("y", legendHeight - 35)
                .style("text-anchor", "end")
                .style("font-size", "12px");

});

//Returns a new array, which a data point per day, except for 29th Feb.
        //Where a day is missing in the original data, use the average from the preceding and next day.
        function createDataPerDay(data) {

let day = data[0].date;
            let lastDay = data[data.length - 1].date;
            let newData = []

let i = 0;

for (day; day <= lastDay; day.setDate(day.getDate() + 1)) {

//skip 29th Feb in new dataset
                if (dateFormat(day) != "2902") {

let datum = {};
                    datum.date = new Date(day);
                    let compareDate = new Date(data[i].date);

if (compareDate.getTime() === day.getTime()) {

datum.extent = data[i].extent;
                        datum.source = "original";

//move to the next day in the original data, unless it is 29th Feb
                        if (i < (data.length - 1)) {
                            i = (data[i + 1].month == 2 && data[i + 1].day == 29) ? i + 2 : i + 1;
                        }

} else {

datum.extent = (data[i - 1].extent + data[i + 1].extent) / 2;
                        datum.source = "avg";

};

newData.push(datum);

}

return newData;

};

function labelX(angle, radius) {
            // change to clockwise
            let a = 360 - angle
            // start from 12 o'clock
            a = a + 180;
            return radius * Math.sin(a * radians)
        }

function labelY(angle, radius) {
            // change to clockwise
            let a = 360 - angle
            // start from 12 o'clock
            a = a + 180;
            return radius * Math.cos(a * radians)
        }

function convertTextToNumbers(d) {
            let dateString = d.day + "/" + d.month + "/" + d.year
            d.date = dateParse(dateString);
            d.extent = +d.extent;
            d.year = +d.year;
            d.month = +d.month;
            d.day = +d.day;
            d.dayOfYear = +dayOfYear(d.date);
            return d;
        };
    </script>
</body>

https://d3js.org/d3.v4.min.js

https://d3js.org/d3-scale-chromatic.v0.3.min.js