Colors Interpolation

<!DOCTYPE html>
<meta charset="utf-8">
<style>
    h3 {
        margin-left: 40px;
        margin-bottom: 2px;
        font-family: 'Maven Pro', sans-serif;
        font-size: 18px;
        font-weight: bold;
    }
</style>
<body>
    <link href="https://fonts.googleapis.com/css?family=Maven+Pro" rel="stylesheet">    
    <script src="//d3js.org/d3.v4.min.js"></script>
    <script>

var margin = { top: 20, right: 20, bottom: 30, left: 50 };
width = 800 - margin.left - margin.right,
height = 50 - margin.top - margin.bottom;

var x = d3.scaleLinear().range([0, width]);
var y = d3.scaleLinear().range([height, 0]);

var circles_data = function (count, y, radius) {
    var data = [];
    for (var j = 0; j < count; j++) {
        data.push({"id": j, "x": j * 2 * radius, "y": y });
        }
    return data;
}

var create_circles = function (color_func) {

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

data = circles_data(20, 140, 15);

x.domain(d3.extent(data, function (d) { return d.x; })).nice();
    y.domain(d3.extent(data, function (d) { return d.y; })).nice();

svg.selectAll(".dot")
        .data(data)
      .enter().append("circle")
        .attr("r", 15)
        .attr("cx", function (d) { return x(d.x); })
        .attr("cy", function (d) { return y(d.y); })
        .style("fill", function (d) { return color_func(d.id); });
}

var data = [];

// ----------------------------------------------------

var color1 = d3.scaleLinear()
        .domain([1, 20])
        .range(['#d73027', '#1a9850'])
        .interpolate(d3.interpolateHcl); //interpolateHsl interpolateHcl interpolateRgb

d3.select("body").append("h3").html("From red to green - 2 colors interpolation")

create_circles(color1);

// ----------------------------------------------------
var color2 = d3.scaleLinear()
                .domain([1, 10, 20])
                .range(['#d73027', '#fee08b', '#1a9850'])
                .interpolate(d3.interpolateHcl); //interpolateHsl interpolateHcl interpolateRgb

d3.select("body").append("h3").html("From red to green - 3 colors interpolation")

create_circles(color2);

// ----------------------------------------------------
var step = d3.scaleLinear()
             .domain([1, 8])
             .range([1, 20]);

var color3= d3.scaleLinear()
                .domain([1, step(2), step(3), step(4), step(5), step(6), step(7), 20])
                .range(['#d73027', '#f46d43', '#fdae61', '#fee08b', '#d9ef8b', '#a6d96a', '#66bd63', '#1a9850'])
                .interpolate(d3.interpolateHcl); //interpolateHsl interpolateHcl interpolateRgb

d3.select("body").append("h3").html("From red to green - 8 colors interpolation")

create_circles(color3);

// ----------------------------------------------------

</script>
</body>

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