Good idea!
Whith last beta http://beta.plantuml.net/plantuml.jar you can have
@startgantt
Project starts the 5th of november 2018
saturday are closed
sunday are closed
[Prototype design] lasts 3 weeks
then [Prototype completed] lasts 5 days
then [Test prototype] lasts 5 days
[Prototype completed] is deleted
@endgantt
This is a first draft, so comment are welcome!