Gradient Legend

Use a linear gradient to make a legend.

<!DOCTYPE html>
<html>
<head>
    <style>
      body {
        margin: 0;
      }
    </style>
</head>
<body>
  <script src="https://d3js.org/d3.v5.min.js"></script>
  <script>
    var data = [{"color":"#000004","value":0},{"color":"#02020c","value":5},{"color":"#050417","value":10},{"color":"#0a0722","value":15},{"color":"#10092d","value":20},{"color":"#160b39","value":25},{"color":"#1e0c45","value":30},{"color":"#260c51","value":35},{"color":"#2f0a5b","value":40},{"color":"#380962","value":45},{"color":"#400a67","value":50},{"color":"#490b6a","value":55},{"color":"#510e6c","value":60},{"color":"#59106e","value":65},{"color":"#61136e","value":70},{"color":"#69166e","value":75},{"color":"#71196e","value":80},{"color":"#781c6d","value":85},{"color":"#801f6c","value":90},{"color":"#88226a","value":95},{"color":"#902568","value":100},{"color":"#982766","value":105},{"color":"#a02a63","value":110},{"color":"#a82e5f","value":115},{"color":"#b0315b","value":120},{"color":"#b73557","value":125},{"color":"#bf3952","value":130},{"color":"#c63d4d","value":135},{"color":"#cc4248","value":140},{"color":"#d34743","value":145},{"color":"#d94d3d","value":150},{"color":"#df5337","value":155},{"color":"#e45a31","value":160},{"color":"#e9612b","value":165},{"color":"#ed6925","value":170},{"color":"#f1711f","value":175},{"color":"#f47918","value":180},{"color":"#f78212","value":185},{"color":"#f98b0b","value":190},{"color":"#fa9407","value":195},{"color":"#fb9d07","value":200},{"color":"#fca60c","value":205},{"color":"#fcb014","value":210},{"color":"#fbba1f","value":215},{"color":"#fac42a","value":220},{"color":"#f8cd37","value":225},{"color":"#f6d746","value":230},{"color":"#f4e156","value":235},{"color":"#f2ea69","value":240},{"color":"#f2f27d","value":245},{"color":"#f5f992","value":250}];
    var extent = d3.extent(data, d => d.value);
    
    var padding = 9;
    var width = 320;
    var innerWidth = width - (padding * 2);
    var barHeight = 8;
    var height = 28;

var xScale = d3.scaleLinear()
        .range([0, innerWidth])
        .domain(extent);

var xTicks = data.filter(f => f.value % 50 === 0).map(d => d.value);

var xAxis = d3.axisBottom(xScale)
        .tickSize(barHeight * 2)
        .tickValues(xTicks);

var svg = d3.select("body").append("svg").attr("width", width).attr("height", height);
    var g = svg.append("g").attr("transform", "translate(" + padding + ", 0)");

var defs = svg.append("defs");
    var linearGradient = defs.append("linearGradient").attr("id", "myGradient");
    linearGradient.selectAll("stop")
        .data(data)
      .enter().append("stop")
        .attr("offset", d => ((d.value - extent[0]) / (extent[1] - extent[0]) * 100) + "%")
        .attr("stop-color", d => d.color);

g.append("rect")
        .attr("width", innerWidth)
        .attr("height", barHeight)
        .style("fill", "url(#myGradient)");

g.append("g")
        .call(xAxis)
      .select(".domain").remove();
  </script>
</body>
</html>

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