[Joker]

Hello again,

I need to draw these kind of things for illustrations in my master thesis, so I upgraded to OmniGraffle Pro.

However, I still don't get the

"I put a triangle in the right place and then used Edit > Shapes > Union to "glue" it onto the arc."

part.

How do you put the triangle in the right place?
When I try it, I find it very difficult to put the tiangle centrical to the line shape.

Is there an easy way to do this?