GlennC

In Designer, you can do this with the Fill tool:
 
1. Create the line in the normal way.
2. With the line selected, select the Fill tool & in the Context toolbar, set the Context to Stroke & Type to Linear.
3. Click on the line near each end where you want the fade to start to create two new stops.
4. In the Color Studio panel or in the popup color selector in the Context toolbar (the item immediately to the right of the Type popup), set the Opacity of the two end stops to zero.
 
You can also vary the midpoints (the lines between the color stops) to control how quickly the fade develops.

You can also use the stroke properties & pressure settings. Change the opacity variance, goto maximum in this case. Assuming you are using a mouse, not a pen and tablet, then go to the pressure setting and make the curve a parabola. Altho' it doesn't give quite the same control as using a gradient, as R C-R describes, it can also be used w. curved lines.
 
For a straight line, you can also use the transparency tool with a radial blend.