digraph g {
  graph [compound = true];
  //presentation
  splines = true;
  overlap = false;
  rankdir = TB;
  subgraph cluster_generators {
    label="generators";
    style="dotted";
    generators_gmail [label="https://www.googleapis.com/gmail/v1"]
  }
  subgraph cluster_providers {
    label="providers";
    providers_gmail [label="GmailProvider"]
  }
  persisters [label="persisters", shape="cylinder"];
  generators_gmail -> providers_gmail
  providers_gmail -> persisters
}