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# Area mark ​

The area mark draws the region between a baseline (x1, y1) and a topline (x2, y2) as in an area chart. Often the baseline represents y = 0, and because the area mark interpolates between adjacent data points, typically both the x and y scales are quantitative or temporal.

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Plot.areaY(aapl, {x: "Date", y: "Close"}).plot()

The area mark has three constructors: areaY for when the baseline and topline share x values, as in a time-series area chart where time goes right→ (or ←left); areaX for when the baseline and topline share y values, as in a time-series area chart where time goes up↑ (or down↓); and lastly the rarely-used area where the baseline and topline share neither x nor y values.

The area mark is often paired with a line and rule mark to accentuate the topline and baseline.

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Plot.plot({
y: {
grid: true
},
marks: [
Plot.areaY(aapl, {x: "Date", y: "Close", fillOpacity: 0.3}),
Plot.lineY(aapl, {x: "Date", y: "Close"}),
Plot.ruleY([0])
]
})

With the default definitions of x = index and y = identity, you can pass an array of numbers as data. Below, a random walk is constructed with d3.cumsum and d3.randomNormal.

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Plot.areaY(d3.cumsum({length: 600}, d3.randomNormal())).plot()

As with lines, points in areas are connected in input order: the first point is connected to the second point, the second is connected to the third, and so on. Area data is typically in chronological order. Unsorted data may produce gibberish.

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Plot.areaY(d3.shuffle(aapl.slice()), {x: "Date", y: "Close"}).plot() // 🌶️

If your data isn’t sorted, use the sort transform.

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Plot.areaY(d3.shuffle(aapl.slice()), {x: "Date", y: "Close", sort: "Date"}).plot()

When the baseline is not y = 0 but instead represents another dimension of data as in a band chart, specify y1 and y2 instead of y.

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Plot.plot({
y: {
label: "Temperature (°F)",
grid: true
},
marks: [
Plot.areaY(sftemp, {x: "date", y1: "low", y2: "high"})
]
})

TIP

Since y1 and y2 refer to different fields here, a y-scale label is specified to improve readability. Also, the band above is spiky; you can smooth it by applying a window transform.

While charts typically put y = 0 on the bottom edge, such that the area grows up↑, this is not required; reversing the y scale will produce a “hanging” area that grows down↓.

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Plot.plot({
x: {
label: null
},
y: {
grid: true,
reverse: true
},
marks: [
Plot.areaY(aapl, {x: "Date", y: "Close", fillOpacity: 0.3}),
Plot.lineY(aapl, {x: "Date", y: "Close"}),
Plot.ruleY([0])
]
})

For a vertically-oriented baseline and topline, such as when time goes up↑ instead of right→, use areaX instead of areaY and swap x and y.

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Plot.plot({
x: {
grid: true
},
marks: [
Plot.areaX(aapl, {y: "Date", x: "Close", fillOpacity: 0.3}),
Plot.lineX(aapl, {y: "Date", x: "Close"}),
Plot.ruleX([0])
]
})

If some channel values are undefined (or null or NaN), gaps will appear between adjacent points. To demonstrate, below we set the y value to NaN for the first three months of each year.

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Plot.plot({
y: {
grid: true
},
marks: [
Plot.areaY(aapl, {x: "Date", y: (d) => d.Date.getUTCMonth() < 3 ? NaN : d.Close, fillOpacity: 0.3}),
Plot.lineY(aapl, {x: "Date", y: (d) => d.Date.getUTCMonth() < 3 ? NaN : d.Close}),
Plot.ruleY([0])
]
})

Supplying undefined values is not the same as filtering the data: the latter will interpolate between the data points. Observe the conspicuous straight lines below!

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Plot.plot({
y: {
grid: true
},
marks: [
Plot.areaY(aapl, {filter: (d) => d.Date.getUTCMonth() >= 3, x: "Date", y: "Close", fillOpacity: 0.3}),
Plot.lineY(aapl, {x: "Date", y: (d) => d.Date.getUTCMonth() < 3 ? NaN : d.Close}),
Plot.ruleY([0])
]
})

If a fill channel is specified, it is assumed to be ordinal or nominal; data is grouped into series and then implicitly stacked.

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Plot.plot({
y: {
transform: (d) => d / 1000,
label: "Unemployed (thousands)"
},
marks: [
Plot.areaY(industries, {x: "date", y: "unemployed", fill: "industry"}),
Plot.ruleY([0])
]
})

CAUTION

This area chart uses color but does not include a legend. This should usually be avoided because color cannot be interpreted without a legend, titles, or labels.

Or, as a streamgraph with the offset stack transform option:

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Plot.plot({
y: {
transform: (d) => d / 1000,
label: "Unemployed (thousands)"
},
marks: [
Plot.areaY(industries, {x: "date", y: "unemployed", fill: "industry", offset: "wiggle"}),
]
})

The z channel determines how data is grouped: if the z channel is not specified, but a varying fill channel is, the fill channel is used for z; the z channel will further fallback to a varying stroke channel if needed.

The z channel (either implicitly or explicitly) is typically used with the stack transform for a stacked area chart or streamgraph. You can disable the implicit stack transform and produce overlapping areas by setting y2 instead of y.

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Plot.plot({
marks: [
Plot.areaY(industries, {x: "date", y2: "unemployed", z: "industry", fillOpacity: 0.1}),
Plot.lineY(industries, {x: "date", y: "unemployed", z: "industry", strokeWidth: 1})
]
})

To vary fill within a single series, set the z option to null.

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Plot.plot({
color: {
type: "log",
legend: true
},
marks: [
Plot.areaY(aapl, {x: "Date", y: "Close", fill: "Volume", z: null}),
Plot.ruleY([0])
]
})

As an alternative to overlapping or stacking, faceting will produce small multiples, here arranged vertically with a shared x-axis.

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Plot.plot({
height: 720,
axis: null,
marks: [
Plot.areaY(industries, {x: "date", y: "unemployed", fy: "industry"}),
Plot.text(industries, Plot.selectFirst({text: "industry", fy: "industry", frameAnchor: "top-left", dx: 6, dy: 6})),
Plot.frame()
]
})

TIP

Above, smaller industries such as agriculture and mining & extraction are dwarfed by larger industries such as wholesale & retail trade. To emphasize each industry’s trend, instead of comparing absolute numbers across industries, you could use the normalize transform.

Or, as a horizon chart, where the area is repeated at different scales with different colors, showing both small-scale variation in position and large-scale variation in color:

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Plot.plot((() => {
const bands = 7;
const step = d3.max(industries, (d) => d.unemployed) / bands;
return {
height: 720,
axis: null,
y: {domain: [0, step]},
color: {scheme: "YlGnBu"},
facet: {data: industries, y: "industry"},
marks: [
d3.range(bands).map((i) => Plot.areaY(industries, {x: "date", y: (d) => d.unemployed - i * step, fill: i, clip: true})),
Plot.text(industries, Plot.selectFirst({text: "industry", frameAnchor: "top-left", dx: 6, dy: 6})),
Plot.frame()
]
};
})())

See also the ridgeline chart example.

Interpolation is controlled by the curve option. The default curve is linear, which draws straight line segments between pairs of adjacent points. A step curve is nice for emphasizing when the value changes, while basis and catmull–rom are nice for smoothing.

## Area options ​

The following channels are required:

• x1 - the horizontal position of the baseline; bound to the x scale
• y1 - the vertical position of the baseline; bound to the y scale

In addition to the standard mark options, the following optional channels are supported:

• x2 - the horizontal position of the topline; bound to the x scale
• y2 - the vertical position of the topline; bound to the y scale
• z - a categorical value to group data into series

If x2 is not specified, it defaults to x1. If y2 is not specified, it defaults to y1. These defaults facilitate sharing x or y coordinates between the baseline and topline. See also the implicit stack transform and shorthand x and y options supported by areaY and areaX.

By default, the data is assumed to represent a single series (i.e., a single value that varies over time). If the z channel is specified, data is grouped by z to form separate series. Typically z is a categorical value such as a series name. If z is not specified, it defaults to fill if a channel, or stroke if a channel.

The stroke defaults to none. The fill defaults to currentColor if the stroke is none, and to none otherwise. If the fill is defined as a channel, the area will be broken into contiguous overlapping segments when the fill color changes; the fill color will apply to the interval spanning the current data point and the following data point. This behavior also applies to the fillOpacity, stroke, strokeOpacity, strokeWidth, opacity, href, title, and ariaLabel channels. When any of these channels are used, setting an explicit z channel (possibly to null) is strongly recommended. The strokeLinecap and strokeLinejoin default to round, and the strokeMiterlimit defaults to 1.

Points along the baseline and topline are connected in input order. Likewise, if there are multiple series via the z, fill, or stroke channel, the series are drawn in input order such that the last series is drawn on top. Typically, the data is already in sorted order, such as chronological for time series; if sorting is needed, consider a sort transform.

The area mark supports curve options to control interpolation between points. If any of the x1, y1, x2, or y2 values are invalid (undefined, null, or NaN), the baseline and topline will be interrupted, resulting in a break that divides the area shape into multiple segments. (See d3-shape’s area.defined for more.) If an area segment consists of only a single point, it may appear invisible unless rendered with rounded or square line caps. In addition, some curves such as cardinal-open only render a visible segment if it contains multiple points.

## areaY(data, options) ​

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Plot.areaY(aapl, {x: "Date", y: "Close"})

Returns a new area with the given data and options. This constructor is used when the baseline and topline share x values, as in a time-series area chart where time goes right→. If neither the y1 nor y2 option is specified, the y option may be specified as shorthand to apply an implicit stackY transform; this is the typical configuration for an area chart with a baseline at y = 0. If the y option is not specified, it defaults to the identity function. The x option specifies the x1 channel; and the x1 and x2 options are ignored.

If the interval option is specified, the binX transform is implicitly applied to the specified options. The reducer of the output y channel may be specified via the reduce option, which defaults to first. To default to zero instead of showing gaps in data, as when the observed value represents a quantity, use the sum reducer.

js
Plot.areaY(observations, {x: "date", y: "temperature", interval: "day"})

The interval option is recommended to “regularize” sampled data; for example, if your data represents timestamped temperature measurements and you expect one sample per day, use "day" as the interval.

The areaY mark draws the region between a baseline (y1) and a topline (y2) as in an area chart. When the baseline is y = 0, the y channel can be specified instead of y1 and y2. For example, here is an area chart of Apple’s stock price.

## areaX(data, options) ​

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Plot.areaX(aapl, {y: "Date", x: "Close"})

Returns a new area with the given data and options. This constructor is used when the baseline and topline share y values, as in a time-series area chart where time goes up↑. If neither the x1 nor x2 option is specified, the x option may be specified as shorthand to apply an implicit stackX transform; this is the typical configuration for an area chart with a baseline at x = 0. If the x option is not specified, it defaults to the identity function. The y option specifies the y1 channel; and the y1 and y2 options are ignored.

If the interval option is specified, the binY transform is implicitly applied to the specified options. The reducer of the output x channel may be specified via the reduce option, which defaults to first. To default to zero instead of showing gaps in data, as when the observed value represents a quantity, use the sum reducer.

js
Plot.areaX(observations, {y: "date", x: "temperature", interval: "day"})

The interval option is recommended to “regularize” sampled data; for example, if your data represents timestamped temperature measurements and you expect one sample per day, use "day" as the interval.

## area(data, options) ​

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Plot.area(aapl, {x1: "Date", y1: 0, y2: "Close"})

Returns a new area with the given data and options. This method is rarely used directly; it is only needed when the baseline and topline have neither common x nor y values. areaY is used in the common horizontal orientation where the baseline and topline share x values, while areaX is used in the vertical orientation where the baseline and topline share y values.

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