Stacked area label placement

Using polylabel plus some extra boundary checks to position labels in a stacked area chart.

<!DOCTYPE html>
<meta charset="utf-8">
<style>
  text {
    font-family: sans-serif;
    font-size: 14px;
    fill: #222;
  }

.axis line, .axis path {
    stroke: #222;
  }

.area text {
    font-size: 18px;
    text-anchor: middle;
  }

.hidden {
    display: none;
  }
</style>
<svg width="960" height="500"></svg>
<script src="//d3js.org/d3.v4.min.js"></script>
<script src="polylabel.min.js"></script>
<script>

var margin = { top: 20, right: 20, bottom: 30, left: 50 },
    width = 960 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom,
    random = d3.randomNormal(0, 3),
    turtles = ["Leonardo", "Donatello", "Raphael", "Michelangelo"],
    colors = ["#ef9a9a", "#9fa8da", "#ffe082", "#80cbc4"];

var svg = d3.select("svg").append("g")
    .attr("transform", "translate(" + margin.left + " " + margin.top + ")");

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

var series = svg.selectAll(".area")
  .data(turtles)
  .enter()
  .append("g")
  .attr("class", "area");

series.append("path")
  .attr("fill", (d, i) => colors[i]);

var xg = svg.append("g")
  .attr("class", "axis x")
  .attr("transform", "translate(0 " + height + ")");

var yg = svg.append("g")
  .attr("class", "axis y");

series.append("text")
  .attr("dy", 5)
  .text(d => d);

var stack = d3.stack().keys(turtles);

var line = d3.line()
  .curve(d3.curveMonotoneX);

randomize();

function randomize() {

var data = [];

for (var i = 0; i < 40; i++) {
    data[i] = {};
    turtles.forEach(function(turtle){
      data[i][turtle] = Math.max(0, random() + (i ? data[i - 1][turtle] : 20));
    });
  }

var stacked = stack(data);

x.domain([0, data.length - 1]);
  y.domain([0, d3.max(stacked[stacked.length - 1].map(d => d[1]))]);

series.data(getPositions(stacked))
    .select("path")
    .attr("d", d => d.path);

series.select("text")
    .classed("hidden", isHidden)
    .attr("x", d => d.label[0])
    .attr("y", d => d.label[1]);

xg.call(d3.axisBottom(x).tickSizeOuter(0));
  yg.call(d3.axisLeft(y).tickSizeOuter(0));

setTimeout(randomize, 750);

}

function getPositions(stacked) {
  return stacked.map(function(area){

var top = area.map((f, j) => [x(j), y(f[1])]),
        bottom = area.map((f, j) => [x(j), y(f[0])]).reverse();

// Exclude the left- and right-most points from the polygon to avoid the edges a bit
    return {
      area: area,
      label: polylabel([top.slice(1, area.length - 1).concat(bottom.slice(1, area.length - 1))]),
      path: line(top) + line(bottom).replace("M", "L") + "Z"
    };

});
}

// Does label fit in its assigned position?
function isHidden(d) {
  var bbox = this.getBBox(),
      labelStart = d.label[0] - bbox.width / 2,
      labelEnd = d.label[0] + bbox.width / 2,
      startIndex,
      endIndex;

// Overlaps the left or right edge
  if (labelEnd > width || labelStart < 0) {
    return true;
  }

// left x value
  startIndex = Math.max(0, Math.floor(x.invert(labelStart)));

// right x value
  endIndex = Math.min(d.area.length - 1, Math.ceil(x.invert(labelEnd)));

for (var i = startIndex; i <= endIndex; i++) {

// Would intersect the bottom
    if (y(d.area[i][0]) < d.label[1] + 5 + bbox.height / 2) {
      return true;
    }

// Would intersect the top
    if (y(d.area[i][1]) > d.label[1] + 5 - bbox.height / 2) {
      return true;
    }
  }

return false;
}

</script>

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