code: func (b *builder) addLegend() {
  	labels := b.config.Labels
  	if len(labels) == 0 {
  		return
  	}
  	title := labels[0]
  	fmt.Fprintf(b, `subgraph cluster_L { "%s" [shape=box fontsize=16`, title)
  	fmt.Fprintf(b, ` label="%s\l"`, strings.Join(labels, `\l`))
  	if b.config.LegendURL != "" {
  		fmt.Fprintf(b, ` URL="%s" target="_blank"`, b.config.LegendURL)
  	}
  	if b.config.Title != "" {
  		fmt.Fprintf(b, ` tooltip="%s"`, b.config.Title)
  	}
  	fmt.Fprintf(b, "] }\n")
  }
docs: // addLegend generates a legend in DOT format.