We are getting too close to GraphViz limits here...
Fortunately, there is a setting to turn around this : skinparam fixCircleLabelOverlapping true
Example :
@startuml
skinparam fixCircleLabelOverlapping true
() "Product Dubbo" as product_dubbo
() "Member Dubbo" as member_dubbo
() "Guide Dubbo" as guide_dubbo
() "Search Dubbo" as search_dubbo
[ME]
[P] - product_dubbo
member_dubbo - [M]
[G] - guide_dubbo
search_dubbo - [S]
guide_dubbo <. [ME]
[ME] ..> member_dubbo
[ME] ..> product_dubbo
[ME] .> search_dubbo
@enduml
http://www.plantuml.com/plantuml/uml/NOv1QeOm44RtESNWgbtK4nGHskAcKi6bI4dCr8Oc6YP9wV5hqYBZln5ytoDlsX0bnUIi25zcyvAaWm_pysHeiVWg5TgtRoGhlJVR2f4IYhjxg0RQTLeYF2UbzWfa0Fz7tdKcnUdHAIIck0AOqIMZaGbh_cVxY9AMJoQ40luDCVKlivY66HvkwZnqZ5D_Q5tMU82_bQLn5eBjy5X32UGFr7LpEV-aPvdZvdfeYvjEplu2