Arquero shorthand

[mbostock]

Currently to use Arquero, you say:

Plot.plot({
  marks: [
    Plot.tickX(table.indices(), {y: table.column("age"), x: table.column("percent")})
  ]
})

But with a little magic, we could support shorthand for Arquero, treating it like an array of objects:

Plot.plot({
  marks: [
    Plot.tickX(table, {y: "age", x: "percent"})
  ]
})

Specifically:

1. If data is an Arquero table, then data is data.indices().
2. If data is an Arquero table, then field names are interpreted as data.column(field).

[Fil]

It seems to me that the shorthand notation already works.

However, it's not optimal in the sense that:

1. Plot calls arrayify on the table (running the iterator)—using table.indices() would avoid that step, but respecting an arquero table as being "arrayified" would work too;
2. Plot instantiates the channels in valueof with Array.from(data, field("fieldname"))—it could just call table.column("field") if it was passed an arquero table.

If we want that optimization, we need a good method to detect an arquero-like table. Maybe when typeof data.column === "function"? Then, arrayify would pass it unchanged, and valueof would call data.column()?

[mbostock]

Sorry, I should have clarified; you’re right that it already works, but I want it to be faster by being equivalent to the longer one.

FWIW, Inputs.table depends typeof data.columnNames === "function", but I think here we’d maybe want to use typeof data.indices === "function" && typeof data.column === "function", if those are the functions we’d use to implement it:

https://github.com/observablehq/inputs/blob/8fbbcf76f40445346637a764825454e4f7fcc76f/src/table.js#L351

[mbostock]

Duplicate of #191.