Hello S.,
For that you can use `style`, like:
@startuml
<style>
arrow {
HorizontalAlignment left
}
</style>
a -> b : long text here,\nwhich is left aligned
<style>
arrow {
HorizontalAlignment center
}
</style>
a <- b : message 2
@enduml
But it seems there are some bugs with:
@startuml
<style>
arrow {
HorizontalAlignment left
FontColor Blue
}
</style>
a -> b : long text here,\nwhich is left aligned
<style>
arrow {
HorizontalAlignment center
FontColor red
}
</style>
a <- b : message 2\nwhy on right!
<style>
arrow {
HorizontalAlignment right
FontColor green
}
</style>
a -> b : long text here,\nwhich is right aligned
@enduml
If that can help,
Regards,
Th.