dot is a language and tool in Graphviz.

Generate a .png graph

dot -Tpng graph.dot -o graph.png

Example graph file: graph.dot

digraph G {
  rankdir = LR;
  node [fontname = "Helvetica"];
  edge [fontname = "Helvetica"];

  user [label = "user"];
  browser [label = "browser"];
  dns [label = "dns server"];
  nginx [label = "nginx"];
  app1 [label = "appserver 1"];
  app2 [label = "appserver 2"];
  app3 [label = "appserver 3"];
  db [label = "db", shape="cylinder"];

  user -> browser;
  browser -> {dns, nginx}
  nginx -> {app1, app2, app3}
  {app1, app2, app3} -> db
}

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