Hello,
Thanks for your contribution. However, we'd better keep the _ for other functionalities, so in V7978, we have implemented this:
@startuml
[bb] --> [zz]
[zz] -> [ee]
[ee] -[bold,#green]-> [yy]
[ww] -[#red]-> [xx]
[ww] -> [aa]
[yy] -[hidden]> [xx]
@enduml
The syntax is more verbose than your proposal, but is more flexible. BTW, this is working with some other diagrams too (Class, State, Usecase...)
Thanks again for your patch.