Hi hogi, that did the trick; thank you. Half the battle for us newbies is knowing what a plot is called (didn't know it was a "vector plot"). Using your info I was able to reproduce the plot in my original post using JMP:
For those who are interested, I have attached a second file with the script for this plot included. Run the script "Vector Plot", right click on the chart and select "Customize..." from the drop-down menu, select "Script" and you'll see what I had to add to make the arrows appear, namely:
Note that the first time I did this there was no Script option, so I pressed the "+" button, which made it appear. After making the addition I pressed OK and voila: the desired arrows.