How to force activity diagram to squash horizontally

Question

Hello, I am building an activity diagram to be placed inside DoxyGen documentation and it is very wide, because it places boxes that are vertically below each other also horizontally next to each other, which I think is awfull waste of space and makes the diagram unnecessarily wide. Is there any way how to force diagram stay as thin as possible?

To help you imagine what I mean, the blocks are arranged like this:

Block
	Block
		Block

But I'd want it to arrange them like this:

	Block
	Block
	Block

Which would safe some horizontal space.

EDIT:
Example of plantuml that result in unsatisfactory diagram layout:

@startuml
title Example

|Line1|
start
|Line3| /' this is here to plot Line3 swimline before Line2'/
|Line2|
if (ondition1) then (true)
|Line3|
    :Do the first thing;
else (false) 
    |Line2|
    if (Condition2) then (true)
    |Line3|
        :Do the second thing;
    |Line1|
    else (false)
        |Line2|
        if (Condition3) then (true)
        |Line3|
            :Do the third thing;
        else (false)
        endif
        :Also do this;
        |Line1|
    endif
endif
|Line1|
:end;
stop
@enduml

Comments

Can you give some plantuml example code that results in the unwanted type of diagram?

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example added to the question