chart = {
const width = 928;
const height = 600;
const marginTop = 20;
const marginRight = 30;
const marginBottom = 30;
const marginLeft = 40;
const x = d3.scaleLinear()
.domain(d3.extent(faithful, d => d.waiting)).nice()
.rangeRound([marginLeft, width - marginRight]);
const y = d3.scaleLinear()
.domain(d3.extent(faithful, d => d.eruptions)).nice()
.rangeRound([height - marginBottom, marginTop]);
const contours = d3.contourDensity()
.x(d => x(d.waiting))
.y(d => y(d.eruptions))
.size([width, height])
.bandwidth(30)
.thresholds(30)
(faithful);
const svg = d3.create("svg")
.attr("width", width)
.attr("height", height)
.attr("viewBox", [0, 0, width, height])
.attr("style", "max-width: 100%; height: auto;");
svg.append("g")
.attr("transform", `translate(0,${height - marginBottom})`)
.call(d3.axisBottom(x).tickSizeOuter(0))
.call(g => g.select(".domain").remove())
.call(g => g.select(".tick:last-of-type text").clone()
.attr("y", -3)
.attr("dy", null)
.attr("font-weight", "bold")
.text("Idle (min.)"));
svg.append("g")
.attr("transform", `translate(${marginLeft},0)`)
.call(d3.axisLeft(y).tickSizeOuter(0))
.call(g => g.select(".domain").remove())
.call(g => g.select(".tick:last-of-type text").clone()
.attr("x", 3)
.attr("text-anchor", "start")
.attr("font-weight", "bold")
.text("Erupting (min.)"));
svg.append("g")
.attr("fill", "none")
.attr("stroke", "steelblue")
.attr("stroke-linejoin", "round")
.selectAll()
.data(contours)
.join("path")
.attr("stroke-width", (d, i) => i % 5 ? 0.25 : 1)
.attr("d", d3.geoPath());
svg.append("g")
.attr("stroke", "white")
.selectAll()
.data(faithful)
.join("circle")
.attr("cx", d => x(d.waiting))
.attr("cy", d => y(d.eruptions))
.attr("r", 2);
return svg.node();
}