I've attached a worksheet that demonstrates a possible approach. It uses a table calculation to compare the position of the last mark on the line with the previous to determine the direction of the arrowhead. I used a dual axis to draw the line layered with the arrow shapes.
It needs to be refined -- for example, you will probably want to add some sophistication to the calculation to add more options for the arrow direction. Hope it helps point you in a possible direction! (Pun most certainly not intended -- I didn't even catch it until I had posted and re-read! )
Arrowhead.twbx.zip 147.0 KB
thanks Josh! it works