Smooth Slider by mbostock

index.html

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

.ticks {
  font: 10px sans-serif;
}

.track,
.track-inset,
.track-overlay {
  stroke-linecap: round;
}

.track {
  stroke: #000;
  stroke-opacity: 0.3;
  stroke-width: 10px;
}

.track-inset {
  stroke: #ddd;
  stroke-width: 8px;
}

.track-overlay {
  pointer-events: stroke;
  stroke-width: 50px;
  cursor: crosshair;
}

.handle {
  fill: #fff;
  stroke: #000;
  stroke-opacity: 0.5;
  stroke-width: 1.25px;
}

</style>
<svg width="960" height="500"></svg>
<script src="//d3js.org/d3.v4.min.js"></script>
<script>

var svg = d3.select("svg"),
    margin = {right: 50, left: 50},
    width = +svg.attr("width") - margin.left - margin.right,
    height = +svg.attr("height");

var hueActual = 0,
    hueTarget = 70,
    hueAlpha = 0.2,
    hueTimer = d3.timer(hueTween);

var x = d3.scaleLinear()
    .domain([0, 180])
    .range([0, width])
    .clamp(true);

var slider = svg.append("g")
    .attr("class", "slider")
    .attr("transform", "translate(" + margin.left + "," + height / 2 + ")");

slider.append("line")
    .attr("class", "track")
    .attr("x1", x.range()[0])
    .attr("x2", x.range()[1])
  .select(function() { return this.parentNode.appendChild(this.cloneNode(true)); })
    .attr("class", "track-inset")
  .select(function() { return this.parentNode.appendChild(this.cloneNode(true)); })
    .attr("class", "track-overlay")
    .call(d3.drag()
        .on("start.interrupt", function() { slider.interrupt(); })
        .on("start drag", function() { hue(x.invert(d3.event.x)); }));

slider.insert("g", ".track-overlay")
    .attr("class", "ticks")
    .attr("transform", "translate(0," + 18 + ")")
  .selectAll("text")
  .data(x.ticks(10))
  .enter().append("text")
    .attr("x", x)
    .attr("text-anchor", "middle")
    .text(function(d) { return d + "°"; });

var handle = slider.insert("circle", ".track-overlay")
    .attr("class", "handle")
    .attr("r", 9);

function hue(h) {
  hueTarget = h;
  hueTimer.restart(hueTween);
}

function hueTween() {
  var hueError = hueTarget - hueActual;
  if (Math.abs(hueError) < 1e-3) hueActual = hueTarget, hueTimer.stop();
  else hueActual += hueError * hueAlpha;
  handle.attr("cx", x(hueActual));
  svg.style("background-color", d3.hsl(hueActual, 0.8, 0.8));
}

</script>