D3 area chart not rendering right

I'm creating a D3 area chart, and the area is showing above the line rather than below.

Area chart with area appearing above line

Here's a fiddle: https://jsfiddle.net/8xsrmgzw/

//Read the data
d3.csv("https://raw.githubusercontent.com/kalidogg/UCB/main/DebtPenny6.csv",

  // When reading the csv, I must format variables:
  function(d){
    return { date : d3.timeParse("%Y-%m-%d")(d.date), value : d.value }
  },

  // Now I can use this dataset:
  function(data) {

    // Add X axis --> it is a date format
    var x = d3.scaleTime()
      .domain(d3.extent(data, function(d) { return d.date; }))
      .range([ 0, width ]);
    svg.append("g")
      .attr("transform", "translate(0," + height + ")")
      .call(d3.axisBottom(x));

    // Add Y axis
    var y = d3.scaleLinear()
      .domain([0, d3.max(data, function(d) { return +d.value; })])
      .range([ height, 0 ]);
    svg.append("g")
      .call(d3.axisLeft(y));

    // Add the area
    svg.append("path")
      .datum(data)
      .attr("fill", "#cce5df")
      .attr("stroke", "#69b3a2")
      .attr("stroke-width", 1.5)
      .attr("d", d3.area()
        .x(function(d) { return x(d.date) })
        .y0(y(0))
        .y1(function(d) { return y(d.value) })
        )

})

</script>

Thanks.

Solution

There are a couple issues. First, you need to sort your data by the date before drawing the area. Second, your dataset contains null values. In the example below, I have filtered those out, but you could consider doing something like these examples to indicate where data is missing.

Also, setting a stroke color on the area will create an outline of the area, including on the sides and bottom. If you just want a line outlining the top of the area, then you can add a separate path and use area.lineY1() to get a line generator for the top line of the area.

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <script src="https://d3js.org/d3.v7.js"></script>
</head>

<body>
    <div id="chart"></div>

    <script>

      d3.csv("https://raw.githubusercontent.com/kalidogg/UCB/main/DebtPenny6.csv", function(d) {
        return { date : d3.timeParse("%Y-%m-%d")(d.date), value : +d.value }
      }).then(chart);

      function chart(data) {
        // set up

        const margin = { top: 10, bottom: 30, left: 50, right: 10};

        const width = 600 - margin.left - margin.right;
        const height = 200 - margin.top - margin.bottom;

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

        const g = svg.append('g')
            .attr('transform', `translate(${margin.left},${margin.top})`);

        // filter and sort data

        data = data
            .filter(d => !isNaN(d.value))
            .sort((a, b) => d3.ascending(a.date, b.date));

        // scales

        const x = d3.scaleTime()
            .domain(d3.extent(data, d => d.date))
            .range([0, width]);
        
        const y = d3.scaleLinear()
            .domain([0, d3.max(data, d => d.value)])
            .range([height, 0]);

        // area generator

        const area = d3.area()
            .x(d => x(d.date))
            .y0(y(0))
            .y1(d => y(d.value));

        // Add the area and top line

        g.append("path")
            .datum(data)
            .attr("fill", "#cce5df")
            .attr("d", area);
       
        g.append("path")
            .datum(data)
            .attr("stroke", "#69b3a2")
            .attr("stroke-width", 1.5)
            .attr("fill", "none")
            .attr("d", area.lineY1());
       
        // Axes
        
        g.append("g")
            .attr("transform", `translate(0,${height})`)
            .call(d3.axisBottom(x));
        
        g.append("g")
            .call(d3.axisLeft(y)
                    .tickFormat(d3.format('~s'))
                    .tickSizeOuter(0));
      }
    </script>
</body>
</html>