The brush mark renders a two-dimensional brush that allows the user to select a rectangular region by clicking and dragging. It is typically used to highlight a subset of data, or to filter data for display in a linked view.
:::plot hidden
Plot.plot({
marks: ((brush) => (d3.timeout(() => brush.move({x1: 36, x2: 48, y1: 15, y2: 20})), [
Plot.dot(penguins, {x: "culmen_length_mm", y: "culmen_depth_mm", stroke: "species"}),
brush
]))(Plot.brush())
}):::
Plot.plot({
marks: [
Plot.dot(penguins, {x: "culmen_length_mm", y: "culmen_depth_mm", stroke: "species"}),
Plot.brush()
]
})The brush mark does not require data. When added to a plot, it renders a brush overlay covering the frame. The user can click and drag to create a rectangular selection, drag the selection to reposition it, or drag an edge or corner to resize it. Clicking outside the selection clears it.
The brushX mark operates on the x axis.
:::plot hidden
Plot.plot({
height: 200,
marks: ((brush) => (d3.timeout(() => brush.move({x1: 3200, x2: 4800})), [
brush,
Plot.dot(penguins, Plot.dodgeY(brush.inactive({x: "body_mass_g", fill: "species"}))),
Plot.dot(penguins, Plot.dodgeY(brush.context({x: "body_mass_g", fill: "#ddd"}))),
Plot.dot(penguins, Plot.dodgeY(brush.focus({x: "body_mass_g", fill: "species"})))
]))(Plot.brushX())
}):::
const brush = Plot.brushX();
Plot.plot({
height: 200,
marks: [
brush,
Plot.dot(penguins, Plot.dodgeY(brush.inactive({x: "body_mass_g", fill: "species"}))),
Plot.dot(penguins, Plot.dodgeY(brush.context({x: "body_mass_g", fill: "#ddd"}))),
Plot.dot(penguins, Plot.dodgeY(brush.focus({x: "body_mass_g", fill: "species"})))
]
})Similarly, the brushY mark operates on the y axis.
The brush dispatches an input event whenever the selection changes. The plot’s value (plot.value) is set to a BrushValue object when a selection is active, or null when the selection is cleared. This allows you to use a plot as an Observable view, or to register an input event listener to react to the brush.
const plot = Plot.plot(options);
plot.addEventListener("input", (event) => {
console.log(plot.value);
});The filter function on the brush value tests whether a data point falls inside the selection. Its signature depends on whether the plot uses faceting, and on the brush’s dimension:
| Facets | 1-D brush | 2-D brush |
|---|---|---|
| none | filter(value) | filter(x, y) |
| fx only | filter(value, fx) | filter(x, y, fx) |
| fy only | filter(value, fy) | filter(x, y, fy) |
| fx and fy | filter(value, fx, fy) | filter(x, y, fx, fy) |
When faceted, the filter returns true only for points in the brushed facet. For example:
plot.addEventListener("input", () => {
const filter = plot.value?.filter;
const selected = filter ? penguins.filter((d) => filter(d.culmen_length_mm, d.culmen_depth_mm)) : penguins;
console.log(selected);
});The brush can be paired with reactive marks that respond to the brush state. Create a brush mark, then call its inactive, context, and focus methods to derive options that reflect the selection.
- inactive — shows the mark when no selection is active; hides it during brushing.
- context — hidden when no selection is active; during brushing, shows points outside the selection.
- focus — hidden when no selection is active; during brushing, shows points inside the selection.
A typical pattern is to layer three reactive marks: the inactive mark provides a default view, while the context and focus marks replace it during brushing, the context dimming unselected points and the focus highlighting selected ones.
:::plot hidden
Plot.plot({
marks: ((brush) => (d3.timeout(() => brush.move({x1: 38, x2: 48, y1: 15, y2: 19})), [
brush,
Plot.dot(penguins, brush.inactive({x: "culmen_length_mm", y: "culmen_depth_mm", fill: "species", r: 2})),
Plot.dot(penguins, brush.context({x: "culmen_length_mm", y: "culmen_depth_mm", fill: "#ccc", r: 2})),
Plot.dot(penguins, brush.focus({x: "culmen_length_mm", y: "culmen_depth_mm", fill: "species", r: 3}))
]))(Plot.brush())
}):::
const brush = Plot.brush();
Plot.plot({
marks: [
brush,
Plot.dot(penguins, brush.inactive({x: "culmen_length_mm", y: "culmen_depth_mm", fill: "species", r: 2})),
Plot.dot(penguins, brush.context({x: "culmen_length_mm", y: "culmen_depth_mm", fill: "#ccc", r: 2})),
Plot.dot(penguins, brush.focus({x: "culmen_length_mm", y: "culmen_depth_mm", fill: "species", r: 3}))
]
}):::tip To achieve higher contrast, you can place the brush before the reactive marks; reactive marks default to using pointerEvents none to ensure they don't obstruct pointer events. :::
The brush mark supports faceting. When the plot uses fx or fy facets, each facet gets its own brush. Starting a brush in one facet clears any selection in other facets. The dispatched value includes the fx and fy facet values of the brushed facet, and the filter function also filters on the relevant facet values.
:::plot hidden
Plot.plot({
height: 270,
grid: true,
marks: ((brush) => (d3.timeout(() => brush.move({x1: 45, x2: 55, y1: 15, y2: 20, fx: "Gentoo"})), [
Plot.frame(),
brush,
Plot.dot(penguins, brush.inactive({x: "culmen_length_mm", y: "culmen_depth_mm", fx: "species", fill: "sex", r: 2})),
Plot.dot(penguins, brush.context({x: "culmen_length_mm", y: "culmen_depth_mm", fx: "species", fill: "#ccc", r: 2})),
Plot.dot(penguins, brush.focus({x: "culmen_length_mm", y: "culmen_depth_mm", fx: "species", fill: "sex", r: 3}))
]))(Plot.brush())
}):::
const brush = Plot.brush();
Plot.plot({
marks: [
Plot.frame(),
brush,
Plot.dot(penguins, brush.inactive({x: "culmen_length_mm", y: "culmen_depth_mm", fx: "species", fill: "sex", r: 2})),
Plot.dot(penguins, brush.context({x: "culmen_length_mm", y: "culmen_depth_mm", fx: "species", fill: "#ccc", r: 2})),
Plot.dot(penguins, brush.focus({x: "culmen_length_mm", y: "culmen_depth_mm", fx: "species", fill: "sex", r: 3}))
]
})For plots with a geographic projection, the brush operates in screen space. The brush value’s x1, y1, x2, y2 bounds are expressed in pixels from the top-left corner of the frame, and the filter function takes the data's coordinates (typically longitude and latitude) and projects them to test against the brush extent.
:::plot hidden
Plot.plot({
projection: "equal-earth",
marks: ((brush) => (d3.timeout(() => brush.move({x1: 300, x2: 500, y1: 50, y2: 200})), [
Plot.geo(land, {strokeWidth: 0.5}),
Plot.sphere(),
brush,
Plot.dot(cities, brush.inactive({x: "longitude", y: "latitude", r: 2, fill: "#999"})),
Plot.dot(cities, brush.context({x: "longitude", y: "latitude", r: 1, fill: "#999"})),
Plot.dot(cities, brush.focus({x: "longitude", y: "latitude", r: 3, fill: "red"}))
]))(Plot.brush())
}):::
const brush = Plot.brush();
Plot.plot({
projection: "equal-earth",
marks: [
Plot.geo(land, {strokeWidth: 0.5}),
Plot.sphere(),
brush,
Plot.dot(cities, brush.inactive({x: "longitude", y: "latitude", r: 2, fill: "#999"})),
Plot.dot(cities, brush.context({x: "longitude", y: "latitude", r: 1, fill: "#999"})),
Plot.dot(cities, brush.focus({x: "longitude", y: "latitude", r: 3, fill: "red"}))
]
})The brush value dispatched on input events. When the brush is cleared, the value is null; otherwise it's an object with the following properties:
- x1 - the lower x bound of the selection (in data space, or pixels if projected)
- x2 - the upper x bound of the selection
- y1 - the lower y bound of the selection
- y2 - the upper y bound of the selection
- fx - the fx facet value, if applicable
- fy - the fy facet value, if applicable
- filter - a function to test whether a point is inside the selection
- pending -
trueduring interaction; absent when committed
By convention, x1 < x2 and y1 < y2. The brushX value does not include y1 and y2; similarly, the brushY value does not include x1 and x2.
The pending property indicates the user is still interacting with the brush. To skip intermediate values and react only to committed selections:
plot.addEventListener("input", () => {
if (plot.value?.pending) return;
// handle committed value (null if cleared)
});const brush = Plot.brush()Returns a new brush. The mark exposes the inactive, context, and focus methods for creating reactive marks that respond to the brush state.
Plot.dot(data, brush.inactive({x: "weight", y: "height", fill: "species"}))Returns mark options that show the mark when no brush selection is active, and hide it during brushing. Use this for the default appearance of data before any selection is made.
Plot.dot(data, brush.context({x: "weight", y: "height", fill: "#ccc"}))Returns mark options that hide the mark by default and, during brushing, show only the points outside the selection. Use this for a dimmed background layer.
Plot.dot(data, brush.focus({x: "weight", y: "height", fill: "species"}))Returns mark options that hide the mark by default and, during brushing, show only the points inside the selection. Use this to highlight the selected data.
brush.move({x1: 36, x2: 48, y1: 15, y2: 20})Programmatically sets the brush selection in data space. For a 2D brush, the value must have x1, x2, y1, and y2 properties; for brushX, x1 and x2; for brushY, y1 and y2. For faceted plots, include fx or fy to target a specific facet. Pass null to clear the selection.
brush.move({x1: 3500, x2: 5000}) // brushXbrush.move({x1: 40, x2: 52, y1: 15, y2: 20, fx: "Chinstrap"})brush.move(null)For projected plots, the coordinates are in pixels (consistent with the BrushValue), so you need to project the two corners of the brush beforehand. In the future Plot might expose its projection to facilitate this. Please upvote this issue to help prioritize this feature.
const brush = Plot.brushX()Returns a new horizontal brush mark that selects along the x axis. The available options are:
- interval - an interval to snap the brush to on release; a number for quantitative scales (e.g.,
100), a time interval name for temporal scales (e.g.,"month"), or an object with floor and offset methods
When an interval is set, the selection snaps to interval boundaries on release, and the filter rounds values before testing, for consistency with binned marks using the same interval. (Use the same interval in the bin transform so the brush aligns with bin edges.)
:::plot defer hidden
Plot.plot({
marks: ((brush) => (d3.timeout(() => brush.move({x1: 3500, x2: 5000})), [
Plot.rectY(penguins, Plot.binX({y: "count"}, {x: "body_mass_g", interval: 100, fill: "currentColor", fillOpacity: 0.3})),
brush,
Plot.rectY(penguins, Plot.binX({y: "count"}, brush.focus({x: "body_mass_g", interval: 100}))),
Plot.ruleY([0])
]))(Plot.brushX({interval: 100}))
}):::
const brush = Plot.brushX({interval: 100});
Plot.plot({
marks: [
Plot.rectY(penguins, Plot.binX({y: "count"}, {x: "body_mass_g", interval: 100, fill: "currentColor", fillOpacity: 0.3})),
brush,
Plot.rectY(penguins, Plot.binX({y: "count"}, brush.focus({x: "body_mass_g", interval: 100}))),
Plot.ruleY([0])
]
})The brushX mark does not support projections.
const brush = Plot.brushY()Returns a new vertical brush mark that selects along the y axis. Accepts the same options as brushX.