For the representation of the principal stresses I would suggest to get rid of the arrows. The arrow head is superfluous and unhelpful. Superfluous because the direction of the principal stress is determined only up to a constant (including a sign), and unhelpful because no possible meaning could be given to the direction represented by the arrow. Rather, I would suggest using just straight lines without symbols extending equal distance from the location at which the principal stress direction is displayed. The length of the straight line could represents the magnitude of the principal stress. Also, it should be possible to display all three directions at a point at the same time. Color coding could be used to indicate which is which (maximum, intermediate, minimum).
The direction of principal stress is an important quantity, and it deserves better treatment in the program.
