[enhancement] Expose Mark's data properties like domain

[dgrcode]

Feature Request

I'd like to be able to get properties from my data before the plot is created, so I can use them during plot creation.

For simplicity, let's say I want to increase my Y domain so it has 10 extra units on each side. I'm thinking it would look something like the following:

const lineMark = Plot.line(values)
const [yDomMin, yDomMax] = lineMark.scale('y').domain

const chart = Plot.plot({
  y: {
    domain: [yDomMin - 10, yDomMax + 10] // for example
  },
  marks: [lineMark]
})

Right now I'd have to do some math, that I imagine will be redone during plot creation.

Why

I'll describe the use case in particular that has triggered this request, since I might be missing something that could allow me to do it with the current state of the library.

I want to be able to find the distance between my domain in y, and add an extra 10% of domain on each side. Sort of like padding the domain.

The reason is that I feel the line is the beginning of the data when the line starts exactly on the bottom-left corner. Adding the padding makes me feel I can scroll sideways to see previous data. Not sure if it makes sense, but that's the way I see it ¯\_(ツ)_/¯

Current Workaround

The current solution I've found is to use d3.extent, but that's about extra 12.6 kb (gzipped) that I imagine is already bundled with plot.

Additional workarounds that I've tried:

• Create another plot as "draft" and get the data I need from there. I went a different way because I feel it's too much to create a brand new plot just for this. The code looked more or less like the following:

  const draftChart = Plot.marks(Plot.line(values)).plot()
  const yExtent = draftChart.scale('y').domain
  // the rest of the code would look just as the code using d3.extent

• Try to mutate chart.scale('y').domain after plot creation. This didn't change anything. The object property was updated, but I imagine the plot itself was created before the mutation, so it didn't have any effect. Maybe a plot.update() method would be handy here, which could be related to updating in place? #1022.