Threshold Encoding

This variation of a line chart demonstrates how to use a linear gradient to change the color of a line based on a y-threshold. This technique is similar to the gradient encoding, but with two stops at the same offset. An alternative method is to draw multiple lines with different colors and different clipping regions.

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

body {
  margin: auto;
  width: 960px;
}

text {
  font: 10px sans-serif;
}

.axis path,
.axis line {
  fill: none;
  stroke: #000;
  shape-rendering: crispEdges;
}

.x.axis path {
  display: none;
}

.line {
  fill: none;
  stroke: url(#temperature-gradient);
  stroke-width: 1.5px;
}

</style>
<body>
<script src="//d3js.org/d3.v3.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;

var parseDate = d3.time.format("%Y%m%d").parse;

var x = d3.time.scale()
    .range([0, width]);

var y = d3.scale.linear()
    .range([height, 0]);

var xAxis = d3.svg.axis()
    .scale(x)
    .orient("bottom");

var yAxis = d3.svg.axis()
    .scale(y)
    .orient("left");

var line = d3.svg.line()
    .interpolate("basis")
    .x(function(d) { return x(d.date); })
    .y(function(d) { return y(d.temperature); });

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 + ")");

d3.tsv("data.tsv", function(error, data) {
  if (error) throw error;

data.forEach(function(d) {
    d.date = parseDate(d.date);
    d.temperature = +d.temperature;
  });

x.domain([data[0].date, data[data.length - 1].date]);
  y.domain(d3.extent(data, function(d) { return d.temperature; }));

svg.append("linearGradient")
      .attr("id", "temperature-gradient")
      .attr("gradientUnits", "userSpaceOnUse")
      .attr("x1", 0).attr("y1", y(50))
      .attr("x2", 0).attr("y2", y(60))
    .selectAll("stop")
      .data([
        {offset: "0%", color: "black"},
        {offset: "50%", color: "black"},
        {offset: "50%", color: "red"},
        {offset: "100%", color: "red"}
      ])
    .enter().append("stop")
      .attr("offset", function(d) { return d.offset; })
      .attr("stop-color", function(d) { return d.color; });

svg.append("g")
      .attr("class", "x axis")
      .attr("transform", "translate(0," + height + ")")
      .call(xAxis);

svg.append("g")
      .attr("class", "y axis")
      .call(yAxis)
    .append("text")
      .attr("transform", "rotate(-90)")
      .attr("y", 6)
      .attr("dy", ".71em")
      .style("text-anchor", "end")
      .text("Temperature (ºF)");

svg.append("path")
      .datum(data)
      .attr("class", "line")
      .attr("d", line);
});

</script>

https://d3js.org/d3.v3.min.js