multiline with threshold

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<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <script src="https://d3js.org/d3.v5.min.js"></script>
  <style>

body {
      margin:auto;	
      padding: 25px;}

.data path {
      fill: none;
    }

.threshold line {
      stroke: black;
      stroke-dasharray: 2%;
    }

.threshold rect {
      fill: red;
      fill-opacity: 0.05
    }
  </style>
</head>

const chart = data => {

const threshold = 12; 
      const margin = {top: 15, right: 35, bottom: 15, left: 50};

const keys = data.columns.slice(1);
      const result = data.map( d => ({
        id: d[data.columns[0]],
        values: keys.map(key => ({year: key, value: +d[key]}))
      }));

const extent = {
        min: +d3.min(result, row => d3.min(row.values, v => v.value)),
        max: +d3.max(result, row => d3.max(row.values, v => v.value)),
      }

const svg = d3.select("#chart"),
            width = +svg.attr("width") - margin.left - margin.right,
            height = +svg.attr("height") - margin.top - margin.bottom;

const xScale = d3.scalePoint()
      .range([margin.left, width - margin.right])
      .domain(keys);

const yScale = d3.scaleLinear()
      .range([height - margin.bottom, margin.top])
      .domain([0, extent.max])

var line = d3.line()
      .x(d => xScale(d.year))
      .y(d => yScale(d.value));

svg.append("g")
        .classed("axis", true)
        .attr("transform", "translate(0," + (height - margin.bottom) + ")")
        .call(d3.axisBottom(xScale));

svg.append("g")
        .classed("axis", true)
        .attr("transform", "translate(" + margin.left + ",0)")
        .call(d3.axisLeft(yScale));

svg.append('g')
        .classed("data", true)
        .selectAll("path")
        .data(result)
        .join(enter => 
              enter.append('path')
              .attr("d", d => line(d.values))
              .style('stroke', 'url(#threshold)')
             )

drawThreshold(svg, yScale, xScale, height, threshold);
    }

const drawThreshold = (svg, yScale, xScale, height, threshold) => {
      const group = svg.append('g').classed('threshold', true);

group.append('line')
        .attr('x1', xScale.range()[0])
        .attr('x2', xScale.range()[1])
        .attr('y1', yScale(threshold))
        .attr('y2', yScale(threshold))

group
        .append('rect')
        .attr('width', xScale.range()[1] - xScale.range()[0])
        .attr('height', yScale.range()[0] - yScale(threshold))
        .attr('x', xScale.range()[0])
        .attr('y', yScale(threshold))

svg.append('defs').append("linearGradient")
        .attr("id", 'threshold')
        .attr("gradientUnits", "userSpaceOnUse")
        .attr("x1", 0)
        .attr("y1", 0)
        .attr("x2", 0)
        .attr("y2", height)
        .selectAll("stop")
        .data([
        {offset: 0, color: "blue"},
        {offset: yScale(threshold) / height, color: "blue"},
        {offset: yScale(threshold) / height, color: "red"},
        {offset: 1, color: "red"}
      ])
        .join("stop")
        .attr("offset", d => d.offset)
        .attr("stop-color", d => d.color);
    }

</script>
</body>

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