One possibility is

@startuml
title Partition-based Vertical Columns
partition "Column 1" {
(*) --> "Activity 1.1"
"Activity 1.1" --> "Activity 1.2"
"Activity 1.2" --> "Activity 1.3"
}
partition "Column 2" {
(*) --> "Activity 2.1"
"Activity 2.1" --> "Activity 2.2"
}
partition "Column 3" {
(*) --> "Activity 3.1"
"Activity 3.1" --> "Activity 3.2"
"Activity 3.2" --> "Activity 3.3"
"Activity 3.3" --> "Activity 3.4"
}
@enduml
Dick Maley