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With last beta http://beta.plantuml.net/plantuml.jar you can now have :
@startuml skinparam TimingFontSize 22 skinparam TimingFontColor red robust "Flow rate" as rate rate has "35 gpm" as high rate has "15 gpm" as low rate has "0 gpm" as none @0 rate is high @5 rate is none @6 rate is low @enduml