In the Line tab, you can specify the Color, Width, Type and Transparecy for the line object.
For information on defining custom dash lines (Type control), see the Plot Details Line tab Help page.
For the straight line (straight line with arrow) object , you can also set set the Alignment to Vertical or Horizontal, and then set it stretching to Whole Layer, All Layers or Whole Page.
Check this the Set as Default checkbox to apply this tab settings to the currently selected object and to all future objects of the same type.
When you change the transparency percentage for the line, the arrow will be applied to the same transparency percentage, and vice versa.