Published
Edited
Jul 21, 2022
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chart = DifferenceChart(data, {
x: d => d.date,
y1: d => d["New York"],
y2: d => d["San Francisco"],
yLabel: "↑ Temperature (°F)",
colors: d3.reverse(d3.schemeRdBu[3]),
curve: d3.curveStep,
width,
height: 600
})
data = FileAttachment("weather.csv").csv({typed: true})
function DifferenceChart(data, {
x = ([x]) => x, // given d in data, returns the (temporal) x-value
y1 = () => 0, // given d in data, returns the (quantitative) baseline y-value
y2 = ([, y]) => y, // given d in data, returns the (quantitative) y-value
defined, // for gaps in data
curve = d3.curveLinear, // curve generator for the lines
marginTop = 20, // top margin, in pixels
marginRight = 30, // right margin, in pixels
marginBottom = 30, // bottom margin, in pixels
marginLeft = 40, // left margin, in pixels
width = 640, // outer width, in pixels
height = 400, // outer height, in pixels
xType = d3.scaleUtc, // type of x-scale
xDomain, // [xmin, xmax]
xRange = [marginLeft, width - marginRight], // [left, right]
yType = d3.scaleLinear, // type of y-scale
yDomain, // [ymin, ymax]
yRange = [height - marginBottom, marginTop], // [bottom, top]
yFormat, // a format specifier string for the y-axis
yLabel, // a label for the y-axis
colors = d3.schemePiYG[3], // colors for negative and positive differences
stroke = "currentColor", // stroke color of line
strokeLinecap = "round", // stroke line cap of the line
strokeLinejoin = "round", // stroke line join of the line
strokeWidth = 1.5, // stroke width of line, in pixels
strokeOpacity = 1, // stroke opacity of line
} = {}) {
// Compute values.
const X = d3.map(data, x);
const Y1 = d3.map(data, y1);
const Y2 = d3.map(data, y2);
const I = d3.range(data.length);
if (defined === undefined) defined = (d, i) => !isNaN(X[i]) && !isNaN(Y1[i]) && !isNaN(Y2[i]);
const D = d3.map(data, defined);
// Compute default domains.
if (xDomain === undefined) xDomain = d3.extent(X);
if (yDomain === undefined) yDomain = d3.extent([...Y1, ...Y2]);
// Construct scales and axes.
const xScale = xType(xDomain, xRange);
const yScale = yType(yDomain, yRange);
const xAxis = d3.axisBottom(xScale).ticks(width / 80).tickSizeOuter(0);
const yAxis = d3.axisLeft(yScale).ticks(height / 60, yFormat);
// A unique identifier for clip paths (to avoid conflicts).
const uid = `O-${Math.random().toString(16).slice(2)}`;
// Helpers for rendering lines and areas.
const line = (y) => d3.line().defined(i => D[i]).curve(curve).x(i => xScale(X[i])).y(y)(I);
const area = (y0, y1) => d3.area().defined(i => D[i]).curve(curve).x(i => xScale(X[i])).y0(y0).y1(y1)(I);
const svg = d3.create("svg")
.attr("width", width)
.attr("height", height)
.attr("viewBox", [0, 0, width, height])
.attr("style", "max-width: 100%; height: auto; height: intrinsic;");
svg.append("g")
.attr("transform", `translate(${marginLeft},0)`)
.call(yAxis)
.call(g => g.select(".domain").remove())
.call(g => g.selectAll(".tick line").clone()
.attr("x2", width - marginLeft - marginRight)
.attr("stroke-opacity", 0.1))
.call(g => g.append("text")
.attr("x", -marginLeft)
.attr("y", 10)
.attr("fill", "currentColor")
.attr("text-anchor", "start")
.text(yLabel));
svg.append("clipPath")
.attr("id", `${uid}-above`)
.append("path")
.attr("d", area(0, i => yScale(Y1[i])));
svg.append("clipPath")
.attr("id", `${uid}-below`)
.append("path")
.attr("d", area(height, i => yScale(Y1[i])));
svg.append("path")
.attr("clip-path", `url(${new URL(`#${uid}-above`, location)})`)
.attr("fill", colors[colors.length - 1])
.attr("d", area(height, i => yScale(Y2[i])));
svg.append("path")
.attr("clip-path", `url(${new URL(`#${uid}-below`, location)})`)
.attr("fill", colors[0])
.attr("d", area(0, i => yScale(Y2[i])));
svg.append("g")
.attr("transform", `translate(0,${height - marginBottom})`)
.call(xAxis)
.call(g => g.select(".domain").remove());
svg.append("path")
.attr("fill", "none")
.attr("stroke", stroke)
.attr("stroke-width", strokeWidth)
.attr("stroke-linecap", strokeLinecap)
.attr("stroke-linejoin", strokeLinejoin)
.attr("stroke-opacity", strokeOpacity)
.attr("d", line(i => yScale(Y2[i])));
return svg.node();
}
import {howto, altplot} from "@d3/example-components"
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