It seems there is an issue with the arrow head, I'll try to investigate it

for now try running it without it

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v := RTView new.

e1 := (RTEllipse new size: 20) elementOn: 'hello'.

e2 := (RTEllipse new size: 20) elementOn: 'world'.

e1 @ RTDraggable.

e2 @ RTDraggable.

l := RTEdge from: e1 to: e2.

l + (RTLine new color: Color red).

"l + (RTSimpleArrowHead new color: Color red)."

e2 translateBy: 60 @ 80.

v addAll: (Array with: e1 with: e2 with: l ).

e1 @ RTLabelled.

e2 @ RTLabelled.

"Note that the RTLabelled interaction has to be set after having added the element in the view"

l @ (RTLabelled new text: 'lining up!').

v open

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As far as documentation goes, it is still heavily work in progress, so right now Agile Visualization is probably the best source (and of course mailing list).