One version line per document type, and stepwise migration (spec/01 §4).

A document type implements this behavior: its current version, and one pure migration step per version. A document carries its version as the stamp "$version", a string key at the top of its map, in the map form and the JSON form alike. Two types' lines are independent: a new step for one moves no other.

defmodule MyApp.Layout do
  @behaviour Macaw.Layers.Version

  @impl true
  def current, do: 2

  # 1 -> 2: `cols` became `columns`.
  @impl true
  def step(%{"cols" => cols} = doc, 1),
    do: {:ok, doc |> Map.delete("cols") |> Map.put("columns", cols)}

  def step(doc, 1), do: {:ok, doc}
end

Macaw.Layers.Version.migrate(%{"$version" => 1, "cols" => 3}, MyApp.Layout)
#=> {:ok, %{"$version" => 2, "columns" => 3}}

The generic part of visualize's Visualize.Chart.Migration (current/0, readable/1, migrate/1), with the line per type rather than one shared by every type.