D3 how to curve segments when using `line.defined()`

Question

I am using two paths here to indicate the present and missing data. I am drawing the solid line path on top of the dotted line path to achieve this. If I pass the data unaltered to the dottedLinePath it draws a continuous curved line, however, if I add .defined(x => x.value !== null to it, the lines of the defined data become straight with no curve. How can I get the path with line.defined(x => x.value !== null) to follow the same curve?

<div className="position-relative" style={{ width, height }}>
  <svg className="position-absolute" width={width} height={height}>
    <defs>
      <linearGradient id={areaGradientId} x1={0} y1={0} x2={0} y2={1}>
        <stop offset="0%" stopColor={color} stopOpacity={0.25} />
        <stop offset="100%" stopColor={color} stopOpacity={0} />
      </linearGradient>
    </defs>

    <g transform={`translate(${padding.left},${padding.top})`}>
      <path d={area(data) || ''} fill={`url(#${areaGradientId})`} />

      <path
        d={dashedLine(data) || ''}
        fill="none"
        stroke={color}
        strokeWidth={3}
        strokeDasharray="6, 4"
      />
      <path
        d={line(data) || ''}
        fill="none"
        stroke={color}
        strokeWidth={3}
      />

      {data.map(d =>
        !d.value ? (
          <rect
            width={6}
            height={11}
            key={Number(d.start_date)}
            className="text-white"
            x={x(d.start_date)}
            y={y(d.value as number)}
            fill={color}
            stroke="currentColor"
            strokeWidth={2}
          />
        ) : (
          <circle
            key={Number(d.start_date)}
            className="text-white u-mb-16"
            cx={x(d.start_date)}
            cy={y(d.value)}
            r={radius}
            fill={color}
            stroke="currentColor"
            strokeWidth={2}
          />
        ),
      )}
    </g>
  </svg>

Answer 1

It's a bit of a mouthful, but you can give the line a custom stroke-dasharray to draw gaps for missing data. I think the following is generic enough to use for your example as well.

const margin = {
    top: 50,
    right: 50,
    bottom: 50,
    left: 50
  },
  width = window.innerWidth - margin.left - margin.right,
  height = window.innerHeight - margin.top - margin.bottom;

const data = d3.range(10).map(i => ({
  x: i,
  y: Math.random()
}));

var x = d3.scaleLinear()
  .domain([0, data.length - 1])
  .range([0, width]);

var y = d3.scaleLinear()
  .domain([0, 1])
  .range([height, 0]);

var line = d3.line()
  .x(d => x(d.x))
  .y(d => y(d.y))
  .curve(d3.curveMonotoneX);

function getLengthAtPoint(targetX, node, length) {
  let result, currentX = node.getPointAtLength(length).x,
    previousLength;
  while (Math.abs(currentX - targetX) > 1) {
    length += (targetX - currentX);
    currentX = node.getPointAtLength(length).x;
  }
  return length;
}

function getStroke(data) {
  // Get the total length
  const totalLength = this.getTotalLength();

  // We find all places where the next value is undefined
  // And follow that until the first defined value
  const isDefined = d => ![2, 5, 6, 7].includes(d.x);

  let currentLength = 0,
    defined = true,
    strokeDasharray = "";
  data.forEach((d, i) => {
    if (!isDefined(d)) {
      if (defined) {
        // This is the first undefined piece, mark the previous point as
        // the last one
        defined = false;
        const newLength = getLengthAtPoint(x(data[i - 1].x), this, currentLength);

        // Add a dotted line until `newLength`
        while (newLength - currentLength > 10) {
          strokeDasharray += "5 5 ";
          currentLength += 10;
        }

        // We have a few pixels left
        if (newLength - currentLength > 5) {
          // Add an interrupted dotted line
          currentLength += 5;
          strokeDasharray += "5 " + (newLength - currentLength) + " ";
        } else {
          strokeDasharray += (newLength - currentLength) + " 0 ";
        }

        currentLength = newLength;
      }
    } else {
      if (!defined) {
        // This is the first defined piece
        defined = true;
        const newLength = getLengthAtPoint(x(data[i - 1].x), this, currentLength);

        // Add a gap until `newLength`
        strokeDasharray += "0 " + (newLength - currentLength) + " ";
        currentLength = newLength;
      }
    }
  });
  return strokeDasharray;
}

var svg = d3.select("body").append("svg")
  .attr("width", width + margin.left + margin.right)
  .attr("height", height + margin.top + margin.bottom)
  .append("g")
  .attr("transform", "translate(" + margin.left + "," + margin.top + ")");

svg.append("g")
  .attr("class", "x axis")
  .attr("transform", "translate(0," + height + ")")
  .call(d3.axisBottom(x));

svg.append("g")
  .attr("class", "y axis")
  .call(d3.axisLeft(y));

svg.append("path")
  .datum(data)
  .attr("class", "line")
  .attr("d", line)
  .attr("stroke-dasharray", getStroke);

.line {
  fill: none;
  stroke: #ffab00;
  stroke-width: 3;
}

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