build.rb

require "erb"
require "pathname"

DOT_TEMPLATE=<<-END
digraph {
size="20,20";
overlap=false;
sep=0.4;
graph [fontname=Helvetica,fontsize=10];
node [fontname=Helvetica,fontsize=10];
edge [fontname=Helvetica,fontsize=10];
rankdir=TB;

edge [len=1.5];
node[shape=box];
%s
}
END

def slugify(text)
  text.gsub(/[^a-zA-Z0-9_\-]+/, "-")
end

class Context
  def graph(title, dot_source)
    dot_source = DOT_TEMPLATE % dot_source
    filename = slugify(title)
    dot_path = "build/graphs/#{filename}.dot"
    png_path = "build/graphs/#{filename}.png"
    png_path_relative_to_build = "graphs/#{filename}.png"
    File.write(dot_path, dot_source)
    send :`, %{dot #{dot_path} -T png -o build/tmp.png}
    exit(1) unless $?.success?
    if !File.exist?(png_path) || File.read("build/tmp.png") != File.read(png_path)
      puts "Overwriting #{png_path}"
      File.write(png_path, File.read("build/tmp.png"))
    end
    "![%s](%s)" % [title, png_path_relative_to_build]
  end

  def binding
    super
  end
end

path = ARGV.fetch(0)
path = Pathname.new(path).relative_path_from(Pathname.new(Dir.getwd))
content = File.read(path)
context = Context.new
rendered = ERB.new(content).result(context.binding)
File.write("build/#{path}", rendered)

On line 32, why use send :`` rather than than the regular backtick literal/just a call to system`? (Not pedantry, a genuine question.)

to use it, you just put <%= graph "title of graph", %{ dot source here} %> in your markdown