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Class 5-Using the D3 legend

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<!DOCTYPE html>
<meta charset="utf-8">
<title>Class 5</title>
<style>
body {
  position: absolute;
  margin: 0px;
  font: 16px sans-serif;
}
svg {
  background-color: #4682b4;
}
.info {
  color: #000;
  position: absolute;
  top: 450px;
  left: 800px;
}
path {
  fill: #555555;
  stroke: #aaaaaa;
}
path.quake {
  fill: crimson;
}
</style>

<script src="https://d3js.org/d3.v4.min.js"></script>
<script src="https://d3js.org/topojson.v2.min.js"></script>

<body>
<script>
var width = 960, height = 500;
var data; 
  // declare a global variable
var svg = d3.select("body").append("svg")
    .attr("width", width)
    .attr("height", height)
    .on("mousemove", mousemoved);
var layer1 = svg.append("g");
var layer2 = svg.append("g");
var info = d3.select("body").append("div")
    .attr("class", "info");
  
// NEW: Title
svg.append('text')
    .attr('x', width / 2)
    .attr('y', "1.5em")
    .style('font-size', '1.5em')
    .style('text-anchor', 'middle')
    .text('Earthquakes in the last week')

var projection = d3.geoAlbersUsa();
var path = d3.geoPath()
    .projection(projection);
  
// NEW: Create a color scale for the earthquakes
var color = d3.scaleOrdinal(d3.schemeCategory10);
  
// Add a legend
var legend = layer2.selectAll('.legend')
    .data(d3.range(10))
  .enter().append('g')
    .attr('class', 'legend')
    .attr('transform', 'translate(' + [width - 150, height / 2] +')');
legend.append('circle')
    .attr('cy', function(d) { return 4 * d * path.pointRadius() ; })
    .attr('r', path.pointRadius())
    .attr('fill', color)
    .attr('stroke', '#aaaaaa')
legend.append('text')
    .attr('x', '1em')
    .attr('y', function(d) { return 4 * d * path.pointRadius(); })
    .attr('dy', '.3em')
    .text(function(d) { return d + ' - ' + (d + 1); })
var usgs = "https://earthquake.usgs.gov/earthquakes/feed/v1.0/summary/all_week.geojson";
var url = "https://umbcvis.github.io/classes/class-03/us.json"
d3.json(url, plotStates);
d3.json(usgs, plotQuakes);
function plotStates(error, json) {
  if (error) throw error;
  json = json.objects.us.geometries.map(function(d) { return topojson.feature(json, d); })
  layer1.selectAll("path")
      .data(json)
    .enter()
      .append("path")
      .attr("d", path);
}
// NEW: Various updates for magnitude-dependent labels and colors
function plotQuakes(error, json) {
  if (error) throw error;
  var features = json.features.filter(inRange); // Only those in the viewport
  var mags = features.map(function(d) { return d.properties.mag });
  var largest = d3.max(mags);
  layer2.selectAll("path.quake")
      .data(features)
    .enter().append("path")
      .attr("class", "quake")
      .style("fill", magColor)
      .attr("d", path)
  // Remove unnecessary legend elements
  legend.filter(function(d) { return d > largest; }).remove();
  // Update the legend labels to indicate # of earthquakes in range
  legend.select('text')
      .text(function(value) { return value + ' - ' + (value + 1) + " (" +
           mags.filter(function(d) { return d >= value && d < value + 1; }).length + ")";
      });
}
// projection returns null for [lon, lat] that won't appear in the viewport
function inRange(d) {
  return projection(d.geometry.coordinates) !== null;
}
// NEW: Return magnitude-dependent color
function magColor(d) {
  return color(Math.floor(d.properties.mag));
}
function mousemoved() {
  info.text(formatLocation(projection.invert(d3.mouse(this)), projection.scale()));
}
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