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document layouts #713

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document layouts #713

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document layouts
Fil Jan 27, 2022
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no mutation
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Update README.md
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remove custom layout example
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52 changes: 50 additions & 2 deletions README.md
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Expand Up @@ -72,9 +72,9 @@ When drawing a single mark, you can call *mark*.**plot**(*options*) as shorthand
```js
Plot.barY(alphabet, {x: "letter", y: "frequency"}).plot()
```
### Layout options
### Geometry options

These options determine the overall layout of the plot; all are specified as numbers in pixels:
These options determine the overall geometry of the plot; all are specified as numbers in pixels:

* **marginTop** - the top margin
* **marginRight** - the right margin
Expand Down Expand Up @@ -1824,6 +1824,54 @@ Plot.stackX2({y: "year", x: "revenue", z: "format", fill: "group"})

Equivalent to [Plot.stackX](#plotstackxstack-options), except that the **x2** channel is returned as the **x** channel. This can be used, for example, to draw a line at the right edge of each stacked area.

## Layouts

A layout processes the transformed and scaled values of a mark before rendering. A layout might, for example, modify the marks’ positions to avoid occlusion. A layout operates in the representation space (such as pixels and colors, *i.e.* after scales have been applied) rather than data space.

### Dodge

The dodge layout can be applied to any mark that consumes *x* or *y*, such as the Dot, Image, Text and Vector marks.

#### Plot.dodgeY([*layoutOptions*, ]*options*)

```js
Plot.dodgeY({x: "date"})
```

If the marks are arranged along the *x* axis, the dodgeY layout piles them vertically, keeping their *x* position unchanged, and creating a *y* position that avoids overlapping.

#### Plot.dodgeX([*layoutOptions*, ]*options*)

```js
Plot.dodgeX({y: "value"})
```

Equivalent to Plot.dodgeY, but the piling is horizontal, keeping the marks’ *y* position unchanged, and creating an *x* position that avoids overlapping.

The dodge layouts accept the following layout options:

* **padding** — a number of pixels added to the radius of the mark to estimate its size
* **anchor** - the layout’s anchor: one of *middle*, *right*, and *left* (default) for dodgeX, and one of *middle*, *top*, and *bottom* (default) for dodgeY.

#### Custom layouts

When it *options* have a *layout* property, the layout function is called after the data has been faceted and scaled; it receives as inputs the index, scales, values (scaled channels) and dimensions, and the mark as this. It must return the (possibly modified) values.

For example, the following custom layout makes every fill color darker:

```js
Plot.dot(data, {
layout: (index, scales, values, dimension) => {
if (values.fill) {
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This example does nothing out of the box, because values.fill will always be undefined. Perhaps a more realistic example would be better? For example, maybe you could have a helper function…

function darker(key = "fill", amount = 1) {
  return (index, scales, {fill, ...values}, dimension) => {
    fill = fill.slice();
    for (const i of index) fill[i] = d3.rgb(fill[i]).darker(amount);
    return {...values, fill};
  };
}

And then

Plot.dot(penguins, {fill: "body_mass_g", layout: darker()})

I guess even this feels very contrived, since it’d be better to do this on the scale definition. Maybe we should remove the custom layouts section until we can think of a better motivating example?

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layout: contrasting on Plot.text could be short (and fix #540). A good contrasting color not as simple as checking a color's lightness, though, so an official implementation would likely diverge from such an example.

Removing this part for now.

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Choosing a contrasting color sounds interesting and useful. Perhaps the fill input channel specifies the background color (e.g., fill: "imdb_rating" in the Simpsons example), and then the layout outputs a new fill channel where each output color is either light or dark depending on the lightness of the input? Something like:

Plot.text(simpsons, Plot.contrast({
  x: "season",
  y: "number_in_season",
  text: d => d.imdb_rating?.toFixed(1),
  title: "title",
  fill: "imdb_rating"
}))

Though, some sort of magic like this might be better…

Plot.text(simpsons, {
  x: "season",
  y: "number_in_season",
  text: d => d.imdb_rating?.toFixed(1),
  title: "title",
  fill: Plot.contrast("imdb_rating")
})

Which, I guess, would be like a per-channel layout.

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I'll open a new issue and prototype :)

for (const i of index) {
values.fill[i] = d3.rgb(values.fill[i]).darker();
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We shouldn’t use/recommend mutation.

}
}
return values;
}
})
```

## Curves

A curve defines how to turn a discrete representation of a line as a sequence of points [[*x₀*, *y₀*], [*x₁*, *y₁*], [*x₂*, *y₂*], …] into a continuous path; *i.e.*, how to interpolate between points. Curves are used by the [line](#line), [area](#area), and [link](#link) mark, and are implemented by [d3-shape](https://github.com/d3/d3-shape/blob/master/README.md#curves).
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