I've solved this issue already but I wanted to share my solution so that the next person who stumbles across this issue has a faster time to resolution.
I was building maps with the Orion Network Atlas and early on I was able to draw lines between two nodes where I put an arrowhead on one end. Go me! But later, no matter what I tried, the drop-down box for arrowheads was grayed out. I couldn't find anything anywhere in the documentation or online to tell me what I'd done to disable this feature or how to turn it back on.
It turns out that in the bottom left corner of the editor, in the Connection Display Options, I have an option enabled for Show Link Utilization. Since the link utilization isn't one-way, it wouldn't make sense to put an arrowhead on the line so naturally, arrowheads are disabled. When I changed the Display Options from Show Link Utilization to Don't Show Additional Info I was able to add arrowheads to my lines again.
I notice that the feature to make lines solid or dashed, and to change the width of a line was also disabled unless I had Don't Show Additional Info selected.
I hope this post saves someone some time. Happy trouble shooting, all!
[Edit] I discovered later that if I have a line with an arrowhead selected when I change the Connection Display Options to Show Link Utilization, the arrowhead stays on the line even though the line changes color to indicate the link status! This was the original result I was trying to achieve, as I wanted to show a link that pointed to an exit to another map (see attached screen shot).
Message was edited by: Garrison -- post script and screen shot added.
