Line chart with color gradient

Steps:

Make sure you understood how to build a basic line chart with d3.js.

Create a color gradient, adding a linearGradient to the svg element. Read this page to understand how a color gradient works.

When the line is drawn, call the color gradient id instead of a usual color with "url(#line-gradient)".

<!DOCTYPE html>
<meta charset="utf-8">

<!-- Load d3.js -->
<script src="https://d3js.org/d3.v4.js"></script>

<!-- Create a div where the graph will take place -->
<div id="my_dataviz"></div>

<script>

// set the dimensions and margins of the graph
var margin = {top: 10, right: 30, bottom: 30, left: 60},
    width = 460 - margin.left - margin.right,
    height = 400 - margin.top - margin.bottom;

// append the svg object to the body of the page
var svg = d3.select("#my_dataviz")
  .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 + ")");

//Read the data
d3.csv("https://raw.githubusercontent.com/holtzy/data_to_viz/master/Example_dataset/3_TwoNumOrdered_comma.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));

    // Max value observed:
    const max = d3.max(data, function(d) { return +d.value; })

    // Add Y axis
    var y = d3.scaleLinear()
      .domain([0, max])
      .range([ height, 0 ]);
    svg.append("g")
      .call(d3.axisLeft(y));

    // Set the gradient
    svg.append("linearGradient")
      .attr("id", "line-gradient")
      .attr("gradientUnits", "userSpaceOnUse")
      .attr("x1", 0)
      .attr("y1", y(0))
      .attr("x2", 0)
      .attr("y2", y(max))
      .selectAll("stop")
        .data([
          {offset: "0%", color: "blue"},
          {offset: "100%", color: "red"}
        ])
      .enter().append("stop")
        .attr("offset", function(d) { return d.offset; })
        .attr("stop-color", function(d) { return d.color; });

    // Add the line
    svg.append("path")
      .datum(data)
      .attr("fill", "none")
      .attr("stroke", "url(#line-gradient)" )
      .attr("stroke-width", 2)
      .attr("d", d3.line()
        .x(function(d) { return x(d.date) })
        .y(function(d) { return y(d.value) })
        )

})

</script>

Steps:

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