How to do it:
Activate the Profile and Cross Section tool.
Click out points in the scene with the left mouse button.
Finish the line with a right-click.
Your profile line is now created – choose whether you want to display it as a longitudinal profile or a cross section.
Tip: The profile line can snap to existing lines in the scene. Click on the start point and finish with a right-click on the end point to automatically mark the entire stretch.