Allow user to add an arrowhead to the beginning and/or end of a LINE

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right arrow is simple, just modify the line's end text to "<sym>arrowheadBlackRight</sym>". using the same trick for begin text ("<sym>arrowheadBlackLeft</sym>").doesn't work that well though, there's a gap between the arrow and the line:

Doesn't work at all for diagonal lines

This would be very useful on charts which sometimes seem to end at the bottom of a page while there is still another page or more to go. It would tell the player to open the chart.

True, although for that purpose simply adding one of the predefined symbols from the Symbols palette (press "Z" to display) would be sufficient, no need to have it be an actual resizable Line.

I have often seen this accompanied by a diagonal arrow pointing roughly NE and I have seen just the arrow. That's what I am referring to. Those letters and arrow are generally very prominent.

I looked in the master palette but the ones there are very small and there are no parameters that I can find to change their size.

Instead of adding the arrow as its own symbol, then, add it within a text element, using the Special Characters palette (press F2 to display while editing text). Then you can size it how you like. And there are quite a few different choices in the Arrows section of the Unicode tab in that dialog. I've dragged a number of these to my own custom Annotations palette.

Thanks.