Crossing the streams

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8" />
  <style>

path {
      fill: none;
      stroke-width: 2px;
      stroke-linejoin: round;
      stroke: #444;
    }

.red path {
      fill: #ba3e2d;
    }

.orange path {
      fill: #ff7d3e;
    }

.teal path {
      fill: #83dfc3;
    }

.blue path {
      fill: #4c8da1;
    }

.yellow path {
      fill: #ffea60;
    }

</style>
</head>
<body>
<div></div>
<script src="
https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.17/d3.min.js"></script>
<script>

var margin = { top: 10, right: 10, left: 10, bottom: 10 },
    width = 960 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom,
    colorOrder = ["teal","red","yellow","orange","blue"];

var line = d3.svg.line();

var svg = d3.select("body").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 + " )");

// Make some random data
var ribbons = d3.range(1980,2005, 0.5).map(function(year,i){
  return({
    id: i, // doesn't matter what this is as long as it's unique
    color: colorOrder[Math.floor(Math.random()*colorOrder.length)],
    startYear: Math.floor(year)
  });
});

// For each year, get an array of ribbons shown, in order (IDs only)
var positionsByYear = d3.range(1980,2005).map(function(year){

return ribbons.filter(function(d){
    return d.startYear <= year; // ignore ribbons that haven't started
  }).sort(sorter).map(function(d){
    return d.id;
  });

});

// Pick the one you want to keep flat
// Could set this manually to the ID you care about
var baselineId = positionsByYear[0][0],
    baselineRow = positionsByYear[positionsByYear.length - 1].indexOf(baselineId);

// Move positions down so the baseline ID is always at the same vertical index
positionsByYear.forEach(function(year){
  while (year.indexOf(baselineId) < baselineRow) {
    year.unshift(null);
  }
});

// Pixel scales
var x = d3.scale.ordinal()
  .domain(d3.range(-1,positionsByYear.length))
  .rangeBands([0,width]);

var y = d3.scale.ordinal()
  .domain(d3.range(ribbons.length + 1))
  .rangeBands([0,height]);

// Construct X/Y coordinate pairs per ribbon
ribbons.forEach(function(ribbon){

var top = [], // Line along the top
      bottom = []; // Line along the bottom

positionsByYear.forEach(function(year,i){

// Vertical position
    var position = year.indexOf(ribbon.id);

if (position >= 0) {

// Add one extra miter-y point at the beginning
      if  (!top.length) {

if (ribbon.id === baselineId) {
          top.push([x(i-1),(y(position) + y(position + 1)) / 2]);
        } else if (position > baselineRow) {

// TODO smarter
          var numBefore = ribbons.filter(function(r){
            var index = year.indexOf(r.id);
            return index > baselineRow && index < position && r.startYear === ribbon.startYear;
          }).length;

top.push([x(i-1),y(position - numBefore)]);

} else {

// TODO smarter
          var numAfter = ribbons.filter(function(r){
            var index = year.indexOf(r.id);
            return index < baselineRow && index > position && r.startYear === ribbon.startYear;
          }).length;

top.push([x(i-1),y(position + 1 + numAfter)]);

}

// Add the top and bottom point
      top.push([x(i),y(position)]);
      bottom.unshift([x(i),y(position + 1)]);
    }

});

// Combine the whole path
  ribbon.positions = top.concat(bottom);

});

var paths = svg.selectAll("g")
  .data(ribbons)
  .enter()
  .append("g")
    .attr("class",function(d){
      return d.color;
    });

paths.append("path")
  .attr("d",function(d){
    return line(d.positions) + "Z";
  });

// What order should the ribbons be stacked in for any given X?
function sorter(a,b){
  var ia = colorOrder.indexOf(a.color),
      ib = colorOrder.indexOf(b.color),
      ga = a.startYear,
      gb = b.startYear;

// sort by color group first
  if (ia !== ib) {
    return ia - ib;
  }

// then within the group
  if (ga !== gb) {
    return ga - gb;
  }

// tiebreaker
  return a.id - b.id;

}

</script>
</body>
</html>