Plot 0.4.0 -> 0.4.1 introduces breaking change where `Plot.scale` rejects parameter objects with an undefined `type` parameter

[MaxwellBo]

Reproduced in a public notebook: https://observablehq.com/@mjbo/plot041-scale-creation-breaking-change.

In Plot 0.4.0

Plot40 = require("@observablehq/plot@0.4.0")
Plot40.scale({ color: { scheme: "viridis" } })
>>> { type: "linear", domain: Array(2), range: Array(2), interpolate: ƒ(e), clamp: false, apply: ƒ(e), invert: ƒ(e)}

In Plot 0.4.1

Plot41 = require("@observablehq/plot@0.4.1")

Plot41.scale({ color: { scheme: "viridis" } })
>>> TypeError: Cannot destructure property 'scale' of 'undefined' as it is undefined.

// explicitly defining the scale `type` suppresses the error
Plot41.scale({ color: { scheme: "viridis", type: "linear" } })
>>> { type: "linear", domain: Array(2), range: Array(2), interpolate: ƒ(e), clamp: false, apply: ƒ(e), invert: ƒ(e)}

[mbostock]

This was an intentional change; see #756. This is described in the release notes:

If scale options are declared, but the scale has no defined type, domain, or data, a scale is no longer constructed rather than a default linear scale.

Our intent is that standalone scale definitions should match the behavior of scale definitions in plots. And, if you specify e.g. color: {legend: true} to a plot definition but the color scale isn’t actually used by any marks, we don’t want to render a spurious color legend. #744

I’ve improved the error message in bc68508.

The fix is, as you discovered, to set the scale type explicitly.

As to whether or not this should be considered a breaking change, I do not think it rises to the level of rescinding the Plot 0.4.1 release. The behavior of Plot.scale given an underspecified scale definition was never fully specified. Also Plot is currently in 0.x in regards to semantic versioning.

That said, I do think Plot.scale(…) should match Plot.plot(…).scale(…), e.g.,

Plot.plot({color: {scheme: "viridis"}}).scale("color")

This variation returns undefined rather than throwing an error, so it would probably be appropriate for Plot.scale (and Plot.legend) to return undefined if no scale definition is provided for symmetry. On the other hand, it does seem appropriate for Plot.scale and Plot.legend to throw in this case if he provided definition is invalid, so I could go either way. I put up #773 for discussion.

[mbostock]

Closing as intended behavior (now with the corrected error message). Thank you for the report.