Class 5 d3.legend

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<!DOCTYPE html>
<meta charset="utf-8">
<title>Class 5 v2</title>
<style>

body {
  position: absolute;
  margin: 0px;
  font: 16px sans-serif;
}

svg {
  background-color: #4682b4;
}

.info {
  font-family: sans-serif;
  color: #000;
  position: absolute;
  top: 450px;
  left: 800px;
}

path {
  fill: #555555;
  stroke: #aaaaaa;
}

/* NEW: style the earthquakes */
path.quake {
  fill: crimson;
}

</style>

<script src="d3.legend.js"></script>

var width = 960, height = 500;

var data;  // declare a global variable

// NEW: Add a color scale
var colors = d3.scaleOrdinal()
        .domain(['< 1', '1-2', '2-3', '3-4', '4-5', '> 5'])
        .range(d3.schemeCategory10)

// NEW: Configure the legend
d3.legend = function() {

// Defaults
  var t = [0, 0],
      cb = null,
      colors = d3.scaleOrdinal()
        .domain(["A", "B", "C", "D"])
        .range(["#98abc5", "#8a89a6", "#7b6888", "#6b486b"]);

var box = 38,
      padding = 2,
      dx = box + padding;

function legend(selection) {
    selection.each(function() {
      var legend = d3.select(this).selectAll(".legend")
        .data( colors.domain().slice().reverse() )
      .enter().append("g")
        .attr("class", "legend")
        .attr("transform", function(d, i) { return "translate(" + t[0] + "," + (t[1] + i * dx) + ")"; });

legend.append("rect")
        .attr("x", 0 )
        .attr("width", box)
        .attr("height", box)
        .style("fill", colors);

legend.append("text")
        .attr("x", - 2 * padding )
        .attr("y", box / 2)
        .attr("dy", ".35em")
        .style("text-anchor", "end")
        .text(function(d) { return d; });

if (typeof cb == "function") cb();
    })
  }

legend.translate = function(_) {
    if (!arguments.length) return t;
    t = _;
    return legend;
  };

legend.colors = function(_) {
    if (!arguments.length) return colors;
    colors = _;
    return legend;
  };

legend.cb = function(_) {
    if (!arguments.length) return cb;
    cb = _;
    return legend;
  };

return legend;
}

var legend = d3.legend()
    .translate([ width - 50 , height / 4 ])
    .colors(colors);

var svg = d3.select("body").append("svg")
    .attr("width", width)
    .attr("height", height)
    .on("mousemove", mousemoved)
    .call(legend);  // NEW: Add the legend to the chart

var layer1 = svg.append("g");
var layer2 = svg.append("g");

// Add a <div> element to label the mouse position
var info = d3.select("body").append("div")
    .attr("class", "info");

var projection = d3.geoAlbersUsa();

var path = d3.geoPath()
    .projection(projection);

// Earthquake data
var usgs = "https://earthquake.usgs.gov/earthquakes/feed/v1.0/summary/all_week.geojson";

// Topojson for US states
var url = "https://umbcvis.github.io/classes/class-03/us.json"

d3.json(url, plotStates);
d3.json(usgs, plotQuakes);

function plotStates(error, json) {

// Convert topojson to an Array of GeoJSON features
  // This defines "data", a global variable that can be manipulated in the developer console
  data = json.objects.us.geometries.map(function(d) { return topojson.feature(json, d); })

// Plot the features (states) -- one <path> element for each state
  layer1.selectAll("path")
      .data(data)
    .enter()
      .append("path")
      .attr("d", path);
}

function plotQuakes(error, json) {
  if (error) return console.log(error);

// Global variable, data, will be visible in the console
  data = json;

// Extract an array of feature objects, one for each earthquake
  var features = json.features;

// Plot the earthquakes
  layer2.selectAll("path.quake")
      .data(features)
    .enter().append("path")
      .attr("class", "quake")
      .attr("d", path)
      .style('fill', quakeColor); // NEW: Add magnitude-dependent color

// NEW: Function returns magnitude-dependent color for earthquakes
  function quakeColor(d) {
    var m = Math.min(Math.floor(Math.abs(d.properties.mag)), colors.domain().length);
    var label = colors.domain()[m];
    return colors(label);
  }
}

// Label the geographic coordinates of the mouse position on the map
function mousemoved() {
  info.text(formatLocation(projection.invert(d3.mouse(this)), projection.scale()));
}

// Format the text that will be displayed in the the <div> element on the map
function formatLocation(p, k) {
  var format = d3.format("." + Math.floor(Math.log(k) / 2 - 2) + "f");
  return (p[1] < 0 ? format(-p[1]) + "°S" : format(p[1]) + "°N") + " "
       + (p[0] < 0 ? format(-p[0]) + "°W" : format(p[0]) + "°E");
}
</script>

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

https://d3js.org/topojson.v2.min.js