The contract a diagram format implements.

A backend receives an AshWorkflow.Charts.Graph and returns the diagram as iodata. It decides only how each node kind and edge kind looks. It does not read the DSL, and it does not write labels, because the graph already carries them.

AshWorkflow.Charts.render/3 accepts a backend module in place of a format name, so a format this library does not ship needs no change here:

defmodule MyApp.PlainTextChart do
  @behaviour AshWorkflow.Charts.Backend

  @impl true
  def render(graph, _opts) do
    for edge <- graph.edges, do: "#{edge.from} -> #{edge.to}: #{edge.label}\n"
  end

  @impl true
  def file_extension, do: "txt"
end

AshWorkflow.Charts.render(MyApp.Candidate, MyApp.PlainTextChart)