I tried implementing a step change by having two values at the same time, but I couldn't seem to bet what I wanted. In the example below, I'm trying to get some step changes followed by a "ramp".

analog "A" between 0 and 100 as A

@A

0 is 50

10 is 50

10 is 0

20 is 0

20 is 10

25 is 10

45 is 90

I workaround appears to be to slightly offset the times of the step-change;

@startuml

analog "A" between 0 and 100 as A

@A

0 is 50

10 is 50

10.1 is 0

20 is 0

20.1 is 10

25 is 10

45 is 90

@enduml

If I offset the time in the other direction, it seems to "mess up" the diagram scaling. I'm not sure why:

@startuml

analog "A" between 0 and 100 as A

@A

0 is 50

9.9 is 50

10 is 0

19.9 is 0

20 is 10

25 is 10

45 is 90

@enduml