Get bandwidth for band scale

[RLesser]

I might be missing something, but it seems that there's no simple way to get the bandwidth used by a band scale. I'm trying to align additional elements to the center of each band, and currently it involves more math than when a band scale is used in D3.

It could be added as a field within here:

[mbostock]

Yes, we should expose the bandwidth and step. 👍

[Fil]

My concern is we'll be exposing them, but they won't be considered as inputs to a new scale, so then someone who tries to build a scale from this description might wonder why it "doesn't work" when they change the values.

Would it be a good idea to show this in some way? By exposing them as info_bandwidth or info_step?

Note that apply() and invert() are already "informative" and not "inputs", so maybe I'm just too cautious.

[mbostock]

I think it’s okay to expose them event though they are only informational. (And we don’t need a separate namespace to highlight that fact; documentation is sufficient.)

[dgrcode]

Where can I find some documentation about the API?

I was looking at this dual-axis bar-line example, and theres a function being used as item in the marks array property, like this:

Plot.plot({
  marks: [
    Plot.barY(data, { x: "year", y: "sales", fill: "#ccc" }),

    (_, { x }, __, dimensions) =>
      Plot.plot({
        // some stuff
      })
  ],

  // some stuff
})

The example mentions

The first experiment uses the internal API and uses the x scale and the dimensions intended for normal marks

Is the internal API documented? if it isn't, what are the files you'd point me to so I can learn more about how to use it?

[Fil]

The marks receive the following arguments to their render function, which is called once for each facet:
index, scales, channels, dimensions, context
for example: https://github.com/observablehq/plot/blob/main/src/marks/area.js#L46

• index is an array of indices (into the channels) that we want to render
• scales is an object with keys corresponding to the scales names (x, etc) and values corresponding to the scaling functions
• channels contains the scaled and transformed values for each channel
• dimensions give the width, height etc, of the facet
• context indicates the document context to use if we want to create a document fragment (a svg:g node, for instance)

This is not documented because we might still want to change it before version 1.0.

[dgrcode]

Awesome, thanks for the info! :)