Absolute spacing between facets

Status Quo

Currently the spacing between facets needs to be set via the implied band scale’s padding option. [1] However, that option only allows adjusting the spacing relative to the band width (or height). [2]

Motivation

For my use case, I’d like to reserve some space between the facets to repeat the x axis between them. Since I know the amount of space the tick labels will need, I’d like to instruct Plot to leave a gap of a certain fixed size.

Problem

With the current API I need to guess the amount of space Plot will use for a single facet and find a fraction that will leave an appropriate amount of space. If Plot decides that a facet should be larger (in the examples below I forced bigger facets), then my space is at risk of being too small or too large.

Examples

The examples are based on [3].

Appropriate spacing

Plot.plot({
  fy: {
    padding: .15
  },
  marks: [    
    Plot.barY(brands, {
      x: "name",
      y: "value",
      fy: (d, i) => i % 3,
    }),
    Plot.axisX({
      facetAnchor: null,
    })
  ],
})

Excessive spacing

Plot.plot({
  fy: {
    padding: .15
  },
  marks: [    
    Plot.barY(brands, {
      x: "name",
      y: "value",
      fy: (d, i) => i % 3,
    }),
    Plot.axisX({
      facetAnchor: null,
    })
  ],
  height: 1000,
})

[1] https://observablehq.com/plot/features/facets#facet-scales
[2] https://observablehq.com/plot/features/scales#discrete-scales
[3] https://observablehq.com/@observablehq/plot-vertical-bars-rotated-labels

Start with the facet scales and discrete scales documentation linked in the issue, then compare the vertical-bars example using fy.padding at different heights. Define how a fixed-size gap should behave when facet dimensions change, and verify that the repeated x-axis use case preserves the requested space.