Thanks for the suggestion!
With last beta http://beta.plantuml.net/plantuml.jar you can have :
[My task1] as [TASK1] lasts 4 days
[My task2] as [TASK2] lasts 6 days
[My task3] as [TASK3] lasts 8 days
[TASK3] starts at [TASK1]'s end
[TASK3] starts at [TASK2]'s end
Is this what you are looking for ?