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HTML

              
                <div id="container">
  <div id="title">
    <h1>Monthly Global Land-Surface Temperature</h1>
    <h2>1753 - 2015</h2>
    <h3>Temperatures are in Celsius and reported as anomalies relative to the Jan 1951-Dec 1980 average.</h3>
    <h3>Estimated Jan 1951-Dec 1980 absolute temperature ℃: 8.66 +/- 0.07</h3>
  </div>
  <div id="chart"></div>
   <div id="notes">
    Code by <a href="http://lukewalker.org" targget="_blank">Luke Walker</a>. This is my solution to a <a href="https://www.freecodecamp.com/challenges/visualize-data-with-a-heat-map" target="_blank">Free Code Camp</a> project. I got my colors from <a href="http://colorbrewer2.org/?type=diverging&scheme=RdYlBu&n=9" target="_blank">ColorBrewer</a> and used a number of tutorials and examples, especially <a href="https://bl.ocks.org/tjdecke/5558084">this one</a> for guidance.
  </div>
</div>
 
              
            
!

CSS

              
                body {
  font: 14px sans-serif;
  shape-rendering: crispEdges;
  padding: 20px;
}

h1 {
  font-size: 30px;
  font-weight: 700;
  margin-bottom: 10px
}

h2 { 
  font-size: 24px;
  margin-bottom: 10px;
}

h3 {
  font-size: 12px;
  margin-bottom: 5px;
}

#container {
  width: 1220px;
  box-shadow: 10px 10px 40px -2px rgba(0,0,0,0.62);
  padding: 20px 0;
  margin: 20px auto;
}

#title {
  width: 100%;
  text-align: center;
}

rect {
  stroke-width: 0px;
}

.axis {
  font-size: 14px;
}

.tooltip {
  position: absolute;
  text-align: center;
  width: 150px;
  height: auto;
  padding: 10px 5px;
  font: 16px sans-serif;
  line-height: 1.4;
  font-weight: 100;
  background: #000;
  border: 0px;
  border-radius: 8px;
  display: none;
  opacity: 0.9;
  color: white;
  
}

.date {
  font-weight: 700;
}


#notes {
  width: 100%;
  text-align: center;
  margin: 20px auto 0 auto;
}
              
            
!

JS

              
                const dataUrl = 'https://raw.githubusercontent.com/FreeCodeCamp/ProjectReferenceData/master/global-temperature.json';

d3.json(dataUrl, function(error, response) {
  if (error) throw error;

  /* Settings */
  const colors =  ['#d73027','#f46d43','#fdae61','#fee090','#ffffbf','#e0f3f8','#abd9e9','#74add1','#4575b4'],
        margin = { 
          top: 0,
          right: 10,
          bottom: 120,
          left: 100
        },
        width = 1200 - margin.left - margin.right,
        height = 700 - margin.top - margin.bottom;

  /* Tooltip */
  const tooltip = d3.select('body').append('div').attr('class', 'tooltip'),
      showTooltip = function(d) {
        tooltip.html('<div class="date">' + formatMonth(d.month) + ', '  + d.year + '</div><div>' + formatNum(baseTemp + d.variance) + '&deg;C</div><div>&Delta; ' + formatNum(d.variance) + '&deg;C</div>')
        .style('left', (d3.event.pageX + 10) + 'px')
        .style('top', (d3.event.pageY + 10) + 'px')
        .style('display', 'block');
      },
      hideTooltip = function(d) {
        tooltip.style('display', 'none');
      };

  /* Data prep */
  const baseTemp = response.baseTemperature,
        data = response.monthlyVariance;
  
  let years = data.map(month => month.year);
  years = years.filter((v, i) => years.indexOf(v) == i);
  
  const minYear = data[0].year,
        maxYear = data[data.length - 1].year,
        variance = data.map(month => month.variance),
        minVariance = d3.min(variance),
        maxVariance = d3.max(variance);
  
  const monthHeight = height / 12,
        monthWidth = width / (variance.length / 12),
        legendWidth = width / 2;
  
  /* Axes & Scales */
  const colorScale = d3.scaleQuantile()
          .domain([minVariance, maxVariance])
          .range(colors.reverse()),
        x = d3.scaleTime()
          .domain([minYear, maxYear])
          .range([0, width]),
        y = d3.scaleTime()
          .domain([12, 1])
          .range([height, height / 12]);
  
  const formatTime = d3.timeFormat('%B'),
        formatMonth = function(month) {
          return formatTime(new Date(2016, month -1))
        },
        formatNum = function(num) { 
          return num.toFixed(3);
        },
        valueLegend = function(color) { 
          return colorScale.invertExtent(color)[0]; 
        };
  
  const xAxis = d3.axisBottom()
    .scale(x)
    .ticks(20)
    .tickFormat(d3.format(''));

  const yAxis = d3.axisLeft()
    .scale(y)
    .tickFormat(formatMonth);
  
  /* Draw heat map */
  const svg = d3.select('#chart').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 + ')');
 
  svg.selectAll('rect')
    .data(data)
    .enter().append('rect')
      .attr('x', function(d) { return x(d.year); })
      .attr('y', function(d) { return y(d.month)})
      .attr('width', monthWidth)
      .attr('height', monthHeight)
      .attr('fill', function(d) {
        return colorScale(d.variance);
       })
       .on('mouseover', showTooltip)
       .on('mouseout', hideTooltip);
  
  svg.append('g')
    .attr('class', 'axis')
    .attr('transform', 'translate(0,' + (height + height / 12) + ')')
    .call(xAxis);
  
  svg.append('g')
    .attr('class', 'axis')
    .attr('transform', 'translate(0,' + (height / 24) + ')')
    .call(yAxis);
  
  const legend = svg.append('g')
    .attr('class', 'legend');
  
  legend.selectAll('rect')
    .data(colors)
    .enter()
    .append('rect')
      .attr('y', height + margin.bottom - 40)
      .attr('x', function(d, i) { return i * (legendWidth / colors.length); })
      .attr('width', legendWidth / colors.length)
      .attr('height', 20)
      .style('fill', function(d) { return d });
   
  legend.selectAll('text')
    .data(colors)
    .enter()
    .append('text')
      .text(function(d) {
        return ((baseTemp + valueLegend(d)).toFixed(1) );
      })
      .attr('text-anchor', 'middle')
      .attr('y', height + margin.bottom - 5)
      .attr('x', function(d, i) { 
        return 15 + i * (legendWidth / colors.length);
      });
}); 
              
            
!
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