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HTML 

              
                <noscript id="no-js">
  <h1>JavaScript is disabled</h1>
  <p>Please enable JavaScript and refresh</p>
</noscript>
!

CSS 

              
                html {
  height: 100%;
}

body {
  min-height: 100%;
  display: flex;
  font-family: Avenir, Helvetica, Arial, sans-serif;
  background-color: LightYellow;
  box-sizing: border-box;
  padding: 8px;
  margin: 0;
}

#container {
  background-color: Ivory;
  text-align: center;
  border: thin solid SlateGray;
  border-radius: 4px;
  box-shadow: 2px 2px 8px;
  margin: auto;
}

#title {
  margin: .5em 0 0 0;
}

#description > h2:first-child {
  margin: .5em 0 0 0;
}
#description > p {
  margin: .5em 0 1em 0;
}

#tooltip {
  position: absolute;
  top: 0;
  let: 0;
  transition: opacity .2s;
  pointer-events: none;
  background-color: rgba(0, 0, 0, .8);
  color: white;
  box-sizing: border-box;
  line-height: 1.4;
  padding: .5em;
  border-radius: .2em;
  backdrop-filter: blur(2px);
}
#tooltip:after {
  content: "";
  position: absolute;
  top: 100%;
  --arrow_size: 5px;
  left: calc(50% - var(--arrow_size));
  border-width: var(--arrow_size);
  border-style: solid;
  border-color: rgba(0, 0, 0, .8) transparent transparent transparent;
}

.cell {
  cursor: pointer;
}
.cell:hover {
  stroke: black;
}

#legend rect {
  stroke: black;
}


#no-js {
  text-align: center;
  margin: auto;
}
!

JS 

              
                const projectName = "heat-map",  // freeCodecamp Testing
      //
      container = d3.select("body")
                    .append("div")
                    .attr("id", "container");

container
  .append("h1")
  .text("Monthly Global Land-Surface Temperature")
  .attr("id", "title");


d3.json("https://raw.githubusercontent.com/freeCodeCamp/ProjectReferenceData/master/global-temperature.json").then(({ monthlyVariance, baseTemperature }) => {
  const description = container
                        .append("div")
                        .attr("id", "description"),
        yearsExtent = d3.extent(monthlyVariance, ({ year }) => year);
  
  description
    .append("h2")
    .text(yearsExtent.join(" ― "));
  
  description
    .append("p")
    .html(`Base Temperature: <strong>${baseTemperature}℃</strong>`);
  
  const svgParent = container
                        .append("div")
                        .style("position", "relative"),  // IMPORTANT
        width = 1260,
        height = 495,
        svg = svgParent
                .append("svg")
                .attr("width", width)
                .attr("height", height),
        //
        tooltip = svgParent
                    .append("div")
                    .attr("id", "tooltip")
                    .style("opacity", 0),  // IMPORTANT
        months = [
          "January",
          "February", 
          "March", 
          "April", 
          "May", 
          "June",
          "July", 
          "August", 
          "September", 
          "October", 
          "November", 
          "December"
        ],
        //
        padding = 70,
        chartTop = height * (18 / 25),
        //
        xScaleRange = [padding, width - padding],
        xScale = d3.scaleTime()
                   .domain(yearsExtent.map(year => new Date(year, 0)))
                   .range(xScaleRange);

  svg
    .append("g")
    .attr("id", "x-axis")
    .attr("transform", `translate(0 ${chartTop})`)
    .call(d3.axisBottom(xScale));
  
  const yScale = d3.scaleBand()
                   .domain(months)
                   .range([0, chartTop]);
  
  svg
    .append("g")
    .attr("id", "y-axis")
    .attr("transform", `translate(${padding} 0)`)
    .call(d3.axisLeft(yScale));
  
  const colors = d3.schemeRdYlBu[11].reverse(),
        colorDomain = d3.extent(monthlyVariance, ({ variance }) => variance),
        getColor = d3.scaleQuantize()
                     .domain(colorDomain)
                     .range(colors);
  
  const dataWidth = (xScaleRange[1] - xScaleRange[0]) / (yearsExtent[1] - yearsExtent[0] + 1),
        dataHeight = yScale.step();
  
  svg
    .selectAll("rect")
    .data(monthlyVariance)
    .join("rect")
    .attr("class", "cell")
    .attr("data-month", ({ month }) => month - 1)
    .attr("data-year", ({ year }) => year)
    .attr("data-temp", ({ variance }) => +(baseTemperature + variance).toFixed(4))
    .attr("width", dataWidth)
    .attr("height", dataHeight)
    .attr("x", ({ year }) => xScale(new Date(year, 0)))
    .attr("y", ({ month }, i, nodes) => yScale(months[nodes[i].getAttribute("data-month")]))
    .attr("fill", ({ variance }) => getColor(variance))
    .on("mouseover", ({ currentTarget }, { year, month, variance }) => {
      tooltip
        .html(`${months[month - 1]} ${year}<br>${currentTarget.getAttribute("data-temp")}℃<br>${variance}℃`)
        .attr("data-year", year)
        .style("opacity", 1)
        .style("left", (+currentTarget.getAttribute("x") - (tooltip.node().offsetWidth - dataWidth) / 2) + "px")
        .style("top", (+currentTarget.getAttribute("y") - tooltip.node().offsetHeight - 6) + "px");
    })
    .on("mouseout", () => tooltip.style("opacity", 0))
  
  const legend = svg
                   .append("g")
                   .attr("id", "legend"),
        legendScaleRange = [padding, 400],
        legendScale = d3.scaleLinear()
                        .domain(colorDomain.map(n => n + baseTemperature))
                        .range(legendScaleRange)
                        .nice(),
        legendTop = height - padding,
        legendDataY = legendTop - dataHeight;
  
  legend
    .append("g")
    .attr("transform", `translate(0 ${legendTop})`)
    .call(d3.axisBottom(legendScale));
  
  legend
    .selectAll("rect")
    .data(colors)
    .join("rect")
    .attr("x", (d, i) => legendScale(getColor.invertExtent(d)[0] + baseTemperature))
    .attr("y", legendDataY)
    .attr("width", d => {
      const [x0, x] = getColor.invertExtent(d);
      return legendScale(x + baseTemperature) - legendScale(x0 + baseTemperature);
    })
    .attr("height", dataHeight)
    .attr("fill", d => d);
  
  legend
    .append("text")
    .text("℃")
    .attr("x", (legendScaleRange[0] + legendScaleRange[1]) / 2)
    .attr("y", legendTop + 35);
  
}).catch(err => {
  container
    .append("h2")
    .text(err);
});
!
999px

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