Formatting links in a diagram
You have several options available to make your links—both the lines and the arrowheads—more distinctive. You might use a patterned link or a dashed line to show a tentative connection between ideas, for example, and a thicker, solid line to show a stronger connection.
After you create a link you like, you can use the Defaults tool on the Formatting toolbar to save the look as the new default. All new links you make will be this style. See Changing the defaults of your current diagram.