digraph G {
    // https://dreampuf.github.io/GraphvizOnline
    rankdir=LR;
    
  subgraph cluster_1 {
    "variable x";
    "variable y";
    "variable z";
    label = "variables and constants";
    color=blue
  }

  subgraph cluster_2 {
    "expression_1aA";
    "expression_1aB";
    "expression_2aA";
    label = "expressions";
    color=red
  }


  "Physics" -> "topic_1";
  "topic_1" -> "subtopic_1a";
  "subtopic_1a" -> "expression_1aA";
  "expression_1aA" -> "variable x"
  "expression_1aA" -> "variable y"
  "subtopic_1a" -> "expression_1aB";
  "expression_1aB" -> "variable z";
  "expression_1aB" -> "variable y";
  
  "Physics" -> "topic_2";
  "topic_2" -> "subtopic_2a";
  "subtopic_2a" -> "expression_2aA";
  "expression_2aA" -> "variable x";
}