You can draw a solid line, then go to Edit > Selection > Properties
Under Stroke color, you can set what is called the opacity using the following Hex code
AA is the opacity, from 00 (100% transparent) to FF (100% opaque)
RR, GG, BB, are the Red, Green and Blue components, also from 00 to FF
In the example below, I make a yellow line with 40% transparency and a line width of 3.