Marks

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If you aren’t yet up and running with Plot, please read our getting started guide first. Tinkering with the code below will give a better sense of how Plot works.

Plot doesn’t have chart types; instead, you construct charts by layering marks.

Marks are geometric shapes

Plot provides a variety of mark types. Think of marks as the “visual vocabulary” — the painter’s palette 🎨, but of shapes instead of colors — that you pull from when composing a chart. Each mark type produces a certain type of geometric shape.

For example, the dot mark draws stroked circles (by default).

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Plot.dot(gistemp, {x: "Date", y: "Anomaly"}).plot()

The line mark draws connected line segments (also known as a polyline or polygonal chain).

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Plot.lineY(gistemp, {x: "Date", y: "Anomaly"}).plot()

And the bar mark draws rectangular bars in either a horizontal (barX→) or vertical (barY↑) orientation.

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Plot.barX(alphabet, {x: "frequency", y: "letter"}).plot()

So instead of looking for a chart type, consider the shape of the primary graphical elements in your chart, and look for the corresponding mark type. If a chart has only a single mark, the mark type is effectively the chart type: the bar mark is used to make a bar chart, the area mark is used to make an area chart, and so on.

Marks are layered

The big advantage of mark types over chart types is that you can compose multiple marks of different types into a single plot. For example, below an area and line are used to plot the same sequence of values, while a rule emphasizes y = 0.

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Plot.plot({
  marks: [
    Plot.ruleY([0]),
    Plot.areaY(aapl, {x: "Date", y: "Close", fillOpacity: 0.2}),
    Plot.lineY(aapl, {x: "Date", y: "Close"})
  ]
})

Each mark supplies its own data; a quick way to combine multiple datasets into a chart is to declare a separate mark for each. You can even use array.map to create multiple marks from nested data.

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Plot.plot({
  marks: [
    [goog, aapl].map((stock) => Plot.lineY(stock, {x: "Date", y: "Close"}))
  ]
})

Marks may also be a function which returns an SVG element, if you wish to insert arbitrary content. (Here we use Hypertext Literal to generate an SVG gradient.)

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Plot.plot({
  marks: [
    () => htl.svg`<defs>
      <linearGradient id="gradient" gradientTransform="rotate(90)">
        <stop offset="15%" stop-color="purple" />
        <stop offset="75%" stop-color="red" />
        <stop offset="100%" stop-color="gold" />
      </linearGradient>
    </defs>`,
    Plot.barY(alphabet, {x: "letter", y: "frequency", fill: "url(#gradient)"}),
    Plot.ruleY([0])
  ]
})

And marks may be null or undefined, which produce no output; this is useful for showing marks conditionally (e.g., when a box is checked).

Show area:

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Plot.plot({
  marks: [
    Plot.ruleY([0]),
    area ? Plot.areaY(aapl, {x: "Date", y: "Close", fillOpacity: 0.2}) : null,
    Plot.lineY(aapl, {x: "Date", y: "Close"})
  ]
})

Marks use scales

Marks are (typically) not positioned in literal pixels, or colored in literal colors, as in a conventional graphics system. Instead you provide abstract values such as time and temperature — marks are drawn “in data space” — and scales encode these into visual values such as position and color. And best of all, Plot automatically creates axes and legends to document the scales’ encodings.

Data is passed through scales automatically during rendering; the mark controls which scales are used. The x and y options are typically bound to the x and y scales, respectively, while the fill and stroke options are typically bound to the color scale. Changing a scale’s definition, say by overriding its domain (the extent of abstract input values) or type, affects the appearance of all marks that use the scale.

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Plot.plot({
  y: {
    type: "log",
    domain: [30, 300],
    grid: true
  },
  marks: [
    Plot.lineY(aapl, {x: "Date", y: "Close"})
  ]
})

Marks have tidy data

A single mark can draw multiple shapes. A mark generally produces a shape — such as a rectangle or circle — for each element in the data.

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

It’s more complicated than that, though, since some marks produce shapes that incorporate multiple data points. Pass the same data to a line and you’ll get a single polyline.

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

And a line mark isn’t even guaranteed to produce a single polyline — there can be multiple polylines, as in a line chart with multiple series (using z).

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Plot.lineY(bls, {x: "date", y: "unemployment", z: "division"}).plot()

Plot favors tidy data structured as an array of objects, where each object represents an observation (a row), and each object property represents an observed value; all objects in the array should have the same property names (the columns).

For example, say we have hourly readings from two sensors A and B. You can represent the sensor log as an array of objects like so:

linedata = [
  {hour: 0, value: 8, sensor: "A"},
  {hour: 0, value: 6, sensor: "B"},
  {hour: 1, value: 7, sensor: "A"},
  {hour: 1, value: 5, sensor: "B"},
  {hour: 2, value: 3, sensor: "A"},
  {hour: 2, value: 0, sensor: "B"},
  {hour: 3, value: 9, sensor: "A"},
  {hour: 3, value: 2, sensor: "B"}
]

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For larger datasets, you can more efficiently pass data using an Apache Arrow table as a columnar data representation. ^0.6.16

Then you can pass the data to the line mark, and extract named columns from the data for the desired options:

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Plot.lineY(linedata, {x: "hour", y: "value", stroke: "sensor"}).plot()

Another common way to extract a column from tabular data is an accessor function. This function is invoked for each element in the data (each row), and returns the corresponding observed value, as with array.map.

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Plot.lineY(linedata, {
  x: (d) => d.hour,
  y: (d) => d.value,
  stroke: (d) => d.sensor
}).plot()

For greater efficiency, Plot also supports columnar data: you can use an Apache Arrow table as data instead of an array of objects. ^0.6.16 You can even pass parallel arrays of values, or Apache Arrow vectors, to each channel.

Plot.lineY({length: linedata.length}, {
  x: linedata.map((d) => d.hour),
  y: linedata.map((d) => d.value),
  stroke: linedata.map((d) => d.sensor)
}).plot()

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Note that when accessor functions or parallel arrays are used instead of field names, automatic axis labels (hour and value) are lost. These can be restored using the label option on the x and y scales.

Marks imply data types

Data comes in different types: quantitative (or temporal) values can be subtracted, ordinal values can be ordered, and nominal (or categorical) values can only be the same or different.

INFO

Because nominal values often need some arbitrary order for display purposes — often alphabetical — Plot uses the term ordinal to refer to both ordinal and nominal data.

Some marks work with any type of data, while other marks have certain requirements or assumptions of data. For example, a line should only be used when both x and y are quantitative or temporal, and when the data is in a meaningful order (such as chronological). This is because the line mark will interpolate between adjacent points to draw line segments. If x or y is nominal — say the names of countries — it doesn’t make sense to use a line because there is no half-way point between two nominal values.

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Plot.lineY(["please", "don’t", "do", "this"]).plot() // 🌶️

WARNING

While Plot aspires to give good defaults and helpful warnings, Plot won’t prevent you from creating a meaningless chart. Only you can prevent bogus charts!

In particular, beware the simple “bar”! A bar mark is used for a bar chart, but a rect mark is needed for a histogram. Plot has four different mark types for drawing rectangles:

• use rect when both x and y are quantitative
• use barX when x is quantitative and y is ordinal
• use barY when x is ordinal and y is quantitative
• use cell when both x and y are ordinal

Plot encourages you to think about data types as you visualize because data types often imply semantics. For example, do you notice anything strange about the bar chart below?

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Plot
  .barY(timeseries, {x: "year", y: "population"}) // 🌶️
  .plot({x: {tickFormat: ""}})

Here’s the underlying data:

timeseries = [
  {year: 2014, population: 7295.290765},
  {year: 2015, population: 7379.797139},
  {year: 2016, population: 7464.022049},
  {year: 2017, population: 7547.858925},
  {year: 2019, population: 7713.468100},
  {year: 2020, population: 7794.798739}
]

The data is missing the population for the year 2018! Because the barY mark implies an ordinal x scale, the gap is hidden. Switching to the rectY mark (with the interval option to indicate that these are annual observations) reveals the missing data.

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Plot
  .rectY(timeseries, {x: "year", y: "population", interval: 1})
  .plot({x: {tickFormat: ""}})

Alternatively, you can keep the barY mark and apply the interval option to the x scale.

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Plot
  .barY(timeseries, {x: "year", y: "population"})
  .plot({x: {tickFormat: "", interval: 1}})

Marks have options

When constructing a mark, you can specify options to change the mark’s appearance. These options are passed as a second argument to the mark constructor. (The first argument is the required data.) For example, if you want filled dots instead of stroked ones, pass the desired color to the fill option:

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Plot.dot(gistemp, {x: "Date", y: "Anomaly", fill: "red"}).plot()

As the name suggests, options are generally optional; Plot tries to provide good defaults for whatever you don’t specify. Plot even has shorthand for various common forms of data. Below, we extract an array of numbers from the gistemp dataset, and use the line mark shorthand to set x = index and y = identity.

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Plot.lineY(gistemp.map((d) => d.Anomaly)).plot()

Some marks even provide default transforms, say for stacking!

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Because Plot strives to be concise, there are many default behaviors, some of which can be subtle. If Plot isn’t doing what you expect, try disabling the defaults by specifying options explicitly.

In addition to the standard options such as fill and stroke that are supported by all mark types, each mark type can support options unique to that type. For example, the dot mark takes a symbol option so you can draw things other than circles. See the documentation for each mark type to see what it supports.

Marks have channels

Channels are mark options that can be used to encode data. These options allow the value to vary with the data, such as a different position or color for each dot. To use a channel, supply it with a column of data, typically as:

• a field (column) name,
• an accessor function, or
• an array of values of the same length and order as the data.

Not all mark options can be expressed as channels. For example, stroke can be a channel but strokeDasharray cannot. This is mostly a pragmatic limitation — it would be harder to implement Plot if every option were expressible as a channel — but it also serves to guide you towards options that are intended for encoding data.

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To vary the definition of a constant option with data, create multiple marks with your different constant options, and then filter the data for each mark to achieve the desired result.

Some options can be either a channel or a constant depending on the provided value. For example, if you set the fill option to purple, Plot interprets it as a literal color.

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Plot
  .barX(timeseries, {x: "population", y: "year", fill: "purple"})
  .plot({y: {label: null, tickFormat: ""}})

Whereas if the fill option is a string but not a valid CSS color, Plot assumes you mean the corresponding column of the data and interprets it as a channel.

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Plot
  .barX(timeseries, {x: "population", y: "year", fill: "year"})
  .plot({y: {label: null, tickFormat: ""}})

If the fill option is a function, it is interpreted as a channel.

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Plot
  .barX(timeseries, {x: "population", y: "year", fill: (d) => d.year})
  .plot({y: {label: null, tickFormat: ""}})

Lastly, note that while channels are normally bound to a scale, you can bypass the color scale here by supplying literal color values to the fill channel.

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Plot
  .barX(timeseries, {x: "population", y: "year", fill: (d) => d.year & 1 ? "red" : "currentColor"})
  .plot({y: {label: null, tickFormat: ""}})

But rather than supplying literal values, it is more semantic to provide abstract values and use scales. In addition to centralizing the encoding definition (if used by multiple marks), it allows Plot to generate a legend.

evenodd

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Plot
  .barX(timeseries, {x: "population", y: "year", fill: (d) => d.year & 1 ? "odd" : "even"})
  .plot({y: {label: null, tickFormat: ""}, color: {legend: true}})

You can then specify the color scale’s domain and range to control the encoding.

Mark options

Mark constructors take two arguments: data and options. Together these describe a tabular dataset and how to visualize it. Option values that must be the same for all of a mark’s generated shapes are known as constants, whereas option values that may vary across a mark’s generated shapes are known as channels. Channels are typically bound to scales and encode abstract data values, such as time or temperature, as visual values, such as position or color. (Channels can also be used to order ordinal domains; see the sort option.)

A mark’s data is most commonly an array of objects representing a tabular dataset, such as the result of loading a CSV file, while a mark’s options bind channels (such as x and y) to columns in the data (such as units and fruit).

sales = [
  {units: 10, fruit: "peach"},
  {units: 20, fruit: "pear"},
  {units: 40, fruit: "plum"},
  {units: 30, fruit: "plum"}
]

Plot.dot(sales, {x: "units", y: "fruit"})

While a column name such as "units" is the most concise way of specifying channel values, values can also be specified as functions for greater flexibility, say to transform data or derive a new column on the fly. Channel functions are invoked for each datum (d) in the data and return the corresponding channel value. (This is similar to how D3’s selection.attr accepts functions, though note that Plot channel functions should return abstract values, not visual values.)

Plot.dot(sales, {x: (d) => d.units * 1000, y: (d) => d.fruit})

Plot also supports columnar data for greater efficiency with bigger datasets; for example, data can be specified as any array of the appropriate length (or any iterable or value compatible with Array.from), and then separate arrays of values can be passed as options.

index = [0, 1, 2, 3]

units = [10, 20, 40, 30]

fruits = ["peach", "pear", "plum", "plum"]

Plot.dot(index, {x: units, y: fruits})

Channel values can also be specified as numbers for constant values, say for a fixed baseline with an area.

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

Missing and invalid data are handled specifically for each mark type and channel. In most cases, if the provided channel value for a given datum is null, undefined, or (strictly) NaN, the mark will implicitly filter the datum and not generate a corresponding output. In some cases, such as the radius (r) of a dot, the channel value must additionally be positive. Plot.line and Plot.area will stop the path before any invalid point and start again at the next valid point, thus creating interruptions rather than interpolating between valid points. Titles will only be added if they are non-empty.

All marks support the following style options:

• fill - fill color
• fillOpacity - fill opacity (a number between 0 and 1)
• stroke - stroke color
• strokeWidth - stroke width (in pixels)
• strokeOpacity - stroke opacity (a number between 0 and 1)
• strokeLinejoin - how to join lines (bevel, miter, miter-clip, or round)
• strokeLinecap - how to cap lines (butt, round, or square)
• strokeMiterlimit - to limit the length of miter joins
• strokeDasharray - a comma-separated list of dash lengths (typically in pixels)
• strokeDashoffset - the stroke dash offset (typically in pixels)
• opacity - object opacity (a number between 0 and 1)
• mixBlendMode - the blend mode (e.g., multiply)
• imageFilter - a CSS filter (e.g., blur(5px)) ^0.6.7
• shapeRendering - the shape-rendering mode (e.g., crispEdges)
• paintOrder - the paint order (e.g., stroke)
• dx - horizontal offset (in pixels; defaults to 0)
• dy - vertical offset (in pixels; defaults to 0)
• target - link target (e.g., “_blank” for a new window); for use with the href channel
• className - the class attribute, if any (defaults to null) ^0.6.16
• ariaDescription - a textual description of the mark’s contents
• ariaHidden - if true, hide this content from the accessibility tree
• pointerEvents - the pointer events (e.g., none)
• clip - whether and how to clip the mark
• tip - whether to generate an implicit pointer tip ^0.6.7

If the clip option is frame (or equivalently true), the mark is clipped to the frame’s dimensions; if the clip option is null (or equivalently false), the mark is not clipped. If the clip option is sphere, then a geographic projection is required and the mark will be clipped to the projected sphere (e.g., the front hemisphere when using the orthographic projection).

If the tip option is true, a tip mark with the pointer transform will be derived from this mark and placed atop all other marks, offering details on demand. If the tip option is set to an options object, these options will be passed to the derived tip mark. If the tip option (or, if an object, its pointer option) is set to x, y, or xy, pointerX, pointerY, or pointer will be used, respectively; otherwise the pointing mode will be chosen automatically. (If the tip mark option is truthy, the title channel is no longer applied using an SVG title element as this would conflict with the tip mark.)

For all marks except text, the dx and dy options are rendered as a transform property, possibly including a 0.5px offset on low-density screens.

All marks support the following optional channels:

• fill - a fill color; bound to the color scale
• fillOpacity - a fill opacity; bound to the opacity scale
• stroke - a stroke color; bound to the color scale
• strokeOpacity - a stroke opacity; bound to the opacity scale
• strokeWidth - a stroke width (in pixels)
• opacity - an object opacity; bound to the opacity scale
• title - an accessible, short-text description (a string of text, possibly with newlines)
• href - a URL to link to
• ariaLabel - a short label representing the value in the accessibility tree

The fill, fillOpacity, stroke, strokeWidth, strokeOpacity, and opacity options can be specified as either channels or constants. When the fill or stroke is specified as a function or array, it is interpreted as a channel; when the fill or stroke is specified as a string, it is interpreted as a constant if a valid CSS color and otherwise it is interpreted as a column name for a channel. Similarly when the fill opacity, stroke opacity, object opacity, stroke width, or radius is specified as a number, it is interpreted as a constant; otherwise it is interpreted as a channel.

The scale associated with any channel can be overridden by specifying the channel as an object with a value property specifying the channel values and a scale property specifying the desired scale name or null for an unscaled channel. For example, to force the stroke channel to be unscaled, interpreting the associated values as literal color strings:

Plot.dot(data, {stroke: {value: "fieldName", scale: null}})

To instead force the stroke channel to be bound to the color scale regardless of the provided values, say:

Plot.dot(data, {stroke: {value: "fieldName", scale: "color"}})

The color channels (fill and stroke) are bound to the color scale by default, unless the provided values are all valid CSS color strings or nullish, in which case the values are interpreted literally and unscaled.

In addition to functions of data, arrays, and column names, channel values can be specified as an object with a transform method; this transform method is passed the mark’s array of data and must return the corresponding array of channel values. (Whereas a channel value specified as a function is invoked repeatedly for each element in the mark’s data, similar to array.map, the transform method is invoked only once being passed the entire array of data.) For example, to pass the mark’s data directly to the x channel, equivalent to Plot.identity:

Plot.dot(numbers, {x: {transform: (data) => data}})

The title, href, and ariaLabel options can only be specified as channels. When these options are specified as a string, the string refers to the name of a column in the mark’s associated data. If you’d like every instance of a particular mark to have the same value, specify the option as a function that returns the desired value, e.g. () => "Hello, world!".

For marks that support the frameAnchor option, it may be specified as one of the four sides (top, right, bottom, left), one of the four corners (top-left, top-right, bottom-right, bottom-left), or the middle of the frame.

All marks support the following transform options:

• filter - apply the filter transform
• sort - apply the sort transform
• reverse - apply the reverse transform
• transform - apply a custom transform
• initializer - apply a custom initializer

The sort option, when not specified as a channel value (such as a field name or an accessor function), can also be used to impute ordinal scale domains.

Insets

Rect-like marks support insets: a positive inset moves the respective side in (towards the opposing side), whereas a negative inset moves the respective side out (away from the opposing side). Insets are specified in pixels using the following options:

• inset - shorthand for all four insets
• insetTop - inset the top edge
• insetRight - inset the right edge
• insetBottom - inset the bottom edge
• insetLeft - inset the left edge

Insets default to zero. Insets are commonly used to create a one-pixel gap between adjacent bars in histograms; the bin transform provides default insets. (Note that the band scale padding defaults to 0.1 as an alternative to insets.)

Rounded corners

Rect-like marks support rounded corners. Each corner (or side) is individually addressable ^0.6.16 using the following options:

• r - the radius for all four corners
• rx1 - the radius for the x1-y1 and x1-y2 corners
• rx2 - the radius for the x2-y1 and x2-y2 corners
• ry1 - the radius for the x1-y1 and x2-y1 corners
• ry2 - the radius for the x1-y2 and x2-y2 corners
• rx1y1 - the radius for the x1-y1 corner
• rx1y2 - the radius for the x1-y2 corner
• rx2y1 - the radius for the x2-y1 corner
• rx2y2 - the radius for the x2-y2 corner
• rx - the x-radius for elliptical corners
• ry - the y-radius for elliptical corners

Corner radii are specified in either pixels or, for rx and ry, as percentages (strings) or the keyword auto. If the corner radii are too big, they are reduced proportionally.

marks(...marks) ^0.2.0

Plot.marks(
  Plot.ruleY([0]),
  Plot.areaY(data, {fill: color, fillOpacity, ...options}),
  Plot.lineY(data, {stroke: color, ...options})
)

A convenience method for composing a mark from a series of other marks. Returns an array of marks that implements the mark.plot function. See the box mark implementation for an example.