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

graph G {
  layout="fdp"

  category1[label=":category1", shape="box"]
  
  subgraph cluster_category1 {
    subcategory1[label=":subcategory1", shape="box"]
    subc2[label=":subc2", shape="box"]
  
    subgraph cluster_subcategory1 {
      subgraph cluster_subcategory1_Array0 {
        article0[label=":article", shape="box"]
        date0[label=":date", shape="box"]
      
        article0 -- "somme article"
        date0 -- "2010-04-04"
      }
      subgraph cluster_subcategory1_Array1 {
        article1[label=":article", shape="box"]
        date1[label=":date", shape="box"]
      
        article1 -- "an other article"
        date1 -- "2010-09-09"
      }
    }
  
    subcategory1 -- cluster_subcategory1
    subc2 -- "..."
  }
  
  category1 -- cluster_category1  
}