# Arrow mark ^0.4.0

TIP

See also the vector mark, which draws arrows of a given length and direction.

The **arrow mark** draws arrows between two points [**x1**, **y1**] and [**x2**, **y2**] in quantitative dimensions. It is similar to the link mark, except it draws an arrowhead and is suitable for directed edges. With the **bend** option, it can be swoopy.⤵︎

For example, below we show the rising inequality (and population) in various U.S. cities from 1980 to 2015. Each arrow represents two observations of a city: the city’s population (**x**) and inequality (**y**) in 1980, and the same in 2015. The arrow’s **stroke** redundantly encodes the change in inequality: red indicates rising inequality, while blue (there are only four) indicates declining inequality.

```
Plot.plot({
grid: true,
inset: 10,
x: {
type: "log",
label: "Population"
},
y: {
label: "Inequality",
ticks: 4
},
color: {
scheme: "BuRd",
label: "Change in inequality from 1980 to 2015",
legend: true,
tickFormat: "+f"
},
marks: [
Plot.arrow(metros, {
x1: "POP_1980",
y1: "R90_10_1980",
x2: "POP_2015",
y2: "R90_10_2015",
bend: true,
stroke: (d) => d.R90_10_2015 - d.R90_10_1980
}),
Plot.text(metros, {
x: "POP_2015",
y: "R90_10_2015",
filter: "highlight",
text: "nyt_display",
fill: "currentColor",
stroke: "white",
dy: -6
})
]
})
```

The arrow mark is also useful for drawing directed graph edges, say representing transition frequencies in a finite state machine.

Fork```
Plot.plot({
inset: 60,
aspectRatio: 1,
axis: null,
marks: [
Plot.dot(markov.nodes, {r: 40}),
Plot.arrow(markov.edges, {
x1: ([[x1]]) => x1,
y1: ([[, y1]]) => y1,
x2: ([, [x2]]) => x2,
y2: ([, [, y2]]) => y2,
bend: true,
strokeWidth: ([,, value]) => value,
strokeLinejoin: "miter",
headLength: 24,
inset: 48
}),
Plot.text(markov.nodes, {text: ["A", "B", "C"], dy: 12}),
Plot.text(markov.edges, {
x: ([[x1, y1], [x2, y2]]) => (x1 + x2) / 2 + (y1 - y2) * 0.15,
y: ([[x1, y1], [x2, y2]]) => (y1 + y2) / 2 - (x1 - x2) * 0.15,
text: ([,, value]) => value
})
]
})
```

For undirected edges, as in the arc diagram of character co-occurrence in *Les Misérables* below, set the **sweep** option to the desired orientation: *-y* for right-bulging links whose endpoints are vertically separated.

```
Plot.plot({
height: 1080,
marginLeft: 100,
axis: null,
x: {domain: [0, 1]}, // see https://github.com/observablehq/plot/issues/1541
color: {domain: d3.range(10), unknown: "#ccc"},
marks: [
Plot.dot(miserables.nodes, {x: 0, y: "id", fill: "group", sort: {y: "fill"}}),
Plot.text(miserables.nodes, {x: 0, y: "id", text: "id", textAnchor: "end", dx: -6, fill: "group"}),
Plot.arrow(miserables.links, {x: 0, y1: "source", y2: "target", sweep: "-y", bend: 90, headLength: 0, stroke: samegroup, sort: samegroup, reverse: true})
]
})
```

## Arrow options

The following channels are required:

**x1**- the starting horizontal position; bound to the*x*scale**y1**- the starting vertical position; bound to the*y*scale**x2**- the ending horizontal position; bound to the*x*scale**y2**- the ending vertical position; bound to the*y*scale

For vertical or horizontal arrows, the **x** option can be specified as shorthand for **x1** and **x2**, and the **y** option can be specified as shorthand for **y1** and **y2**, respectively.

The arrow mark supports the standard mark options. The **stroke** defaults to *currentColor*. The **fill** defaults to *none*. The **strokeWidth** defaults to 1.5, and the **strokeMiterlimit** defaults to 1. The following additional options are supported:

**bend**- the bend angle, in degrees; defaults to 0°; true for 22.5°**headAngle**- the arrowhead angle, in degrees; defaults to 60°**headLength**- the arrowhead scale; defaults to 8**insetEnd**- inset at the end of the arrow (useful if the arrow points to a dot)**insetStart**- inset at the start of the arrow**inset**- shorthand for the two insets**sweep**- the sweep order

The **bend** option sets the angle between the straight line connecting the two points and the outgoing direction of the arrow from the start point. It must be within ±90°. A positive angle will produce a clockwise curve; a negative angle will produce a counterclockwise curve; zero will produce a straight line. The **headAngle** determines how pointy the arrowhead is; it is typically between 0° and 180°. The **headLength** determines the scale of the arrowhead relative to the stroke width. Assuming the default of stroke width 1.5px, the **headLength** is the length of the arrowhead’s side in pixels.

The **sweep** option ^0.6.10 controls the bend orientation. It defaults to 1 indicating a positive (clockwise) bend angle; -1 indicates a negative (anticlockwise) bend angle; 0 effectively clears the bend angle. If *-x*, the bend angle is flipped when the ending point is to the left of the starting point — ensuring all arrows bulge up (down if bend is negative); if *-y*, the bend angle is flipped when the ending point is above the starting point — ensuring all arrows bulge right (left if bend is negative); the sign is negated for *+x* and *+y*.

## arrow(*data*, *options*)

`Plot.arrow(inequality, {x1: "POP_1980", y1: "R90_10_1980", x2: "POP_2015", y2: "R90_10_2015", bend: true})`

Returns a new arrow with the given *data* and *options*.