Drag a Line from the Drawing Elements in the Component Library menu.

With the line selected, change the type in the Object Property menu.

Options include:
Horizontal line
Horizontal with left arrow
Horizontal with right arrow
Horizontal with arrows
Vertical line
Vertical with left arrow
Vertical with right arrow
Vertical with arrows
Upward line
Upward with left arrow
Upward with right arrow
Upward with arrows
Downward line
Downward with left arrow
Downward with right arrow
Downward with arrows  

To create a diagonal line, select upward line and resize the line.

You can also adjust the line thickness and color from Object Property.

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