//{:category1 => 
// {:subcategory1 => 
//  [ 
//   {:article => "some article", :date => "2010-04-04"},
//   ...
//  ],
//  :subc2 => [...] 
// },
// :category2 => {...},
// ...
//}

graph G {
  rankdir=LR
  category1
  
  subgraph clusterC {
    node[shape="box"]
    subcategory1
    subc2
  
    subgraph clusterS {
      article -- "somme article"
      date -- "2010-04-04"
    }
  
    subcategory1 -- clusterS
    subc2 -- "..."
  }
  
  category1 -- clusterC
}