density mark

[mbostock]

Plot.plot({
  inset: 20,
  marks: [
    Plot.density(faithful, {x: "waiting", y: "eruptions", stroke: "steelblue", strokeWidth: 0.25}),
    Plot.density(faithful, {x: "waiting", y: "eruptions", thresholds: 4, stroke: "steelblue"}),
    Plot.dot(faithful, {x: "waiting", y: "eruptions", fill: "currentColor", r: 1.5})
  ]
})

TODO

• group by z (or fill or stroke)

Open questions:

• Should this be called contour instead of density (to match ggplot2)?
• What if you want non-density contours, like for a topographic map or a continuous function?
• What if you want one-dimensional density (KDE)? Is that a transform?
• Should there be a legend or continuous color encoding for density contours?

[Fil]

A few more features in #948.

Should this be called contour instead of density (to match ggplot2)?

I agree that density or density contours is a better name than "contours" because we might want to create other types of contours.

one-dimensional density (KDE)? Is that a transform?

yes, very similar to 1-d binning

a legend or continuous color encoding for density contours?

I don't think it's possible to make a readable "density level" legend (points per pixel square?)

[Fil]

superseded by #948