Legendary Key

This example shows the use of a legend to display a quantized threshold key. Inspiration comes from the need to describe the colors of a Choropleth in limited horizontal space. The legend box allows stacking of blocks to represent the hues. With limited space comes the possible need to split the title into two (or more) lines.

In D.C. Choropleth, I used a variation on Bostock's Threshold Key to show quantized colors. That particular diagram had quite a bit of empty space when displayed in a 960px wide block. With this "legendary" key, it can be shown in a block half as wide.

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Legendary Key</title>
<script src="https://d3js.org/d3.v3.min.js"></script>
<style>

body {
  font: 10px sans-serif;
}

.key path {
  display: none;
}

.key line {
  stroke: #000;
  shape-rendering: crispEdges;
}

.legend-title {
    font-weight: bold;
}

.legend-box {
    fill: none;
    stroke: #888;
    font-size: 10px;
}

</style>
</head>
<body>
<script type="text/javascript">

var width = 960,
    height = 500,
    boxmargin = 4,
    lineheight = 14,
    keyheight = 10,
    keywidth = 40,
    boxwidth = 2 * keywidth,
    formatPercent = d3.format(".0%");
    
var margin = { "left": 160, "top": 80 };

var sevenshadesofgold = [ "#ffffb2", "#fee391", "#fec44f", "#fe9929", "#ec7014", "#cc4c02", "#8c2d04" ];

var title = ['Household owner','occupancy rate'],
    titleheight = title.length*lineheight + boxmargin;
    
var x = d3.scale.linear()
    .domain([0, 1]);
    
var quantize = d3.scale.quantize()
    .domain([0, 1])
    .range(sevenshadesofgold);
    
var ranges = quantize.range().length;

// return quantize thresholds for the key    
var qrange = function(max, num) {
    var a = [];
    for (var i=0; i<num; i++) {
        a.push(i*max/num);
    }
    return a;
}
   
var svg = d3.select("body").append("svg")
    .attr("width", width)
    .attr("height", height);
    
// make legend 
var legend = svg.append("g")
    .attr("transform", "translate ("+margin.left+","+margin.top+")")
    .attr("class", "legend");
    
legend.selectAll("text")
    .data(title)
    .enter().append("text")
    .attr("class", "legend-title")
    .attr("y", function(d, i) { return (i+1)*lineheight-2; })
    .text(function(d) { return d; })

// make legend box 
var lb = legend.append("rect")
    .attr("transform", "translate (0,"+titleheight+")")
    .attr("class", "legend-box")
    .attr("width", boxwidth)
    .attr("height", ranges*lineheight+2*boxmargin+lineheight-keyheight);

// make quantized key legend items
var li = legend.append("g")
    .attr("transform", "translate (8,"+(titleheight+boxmargin)+")")
    .attr("class", "legend-items");

li.selectAll("rect")
    .data(quantize.range().map(function(color) {
      var d = quantize.invertExtent(color);
      if (d[0] == null) d[0] = x.domain()[0];
      if (d[1] == null) d[1] = x.domain()[1];
      return d;
    }))
    .enter().append("rect")
    .attr("y", function(d, i) { return i*lineheight+lineheight-keyheight; })
    .attr("width", keywidth)
    .attr("height", keyheight)
    .style("fill", function(d) { return quantize(d[0]); });
    
li.selectAll("text")
    .data(qrange(quantize.domain()[1], ranges))
    .enter().append("text")
    .attr("x", 48)
    .attr("y", function(d, i) { return (i+1)*lineheight-2; })
    .text(function(d) { return formatPercent(d); });
   
</script>
</body>
</html>

Modified http://d3js.org/d3.v3.min.js to a secure url

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