extending a line, preserving slope
 

When extending the length of a line, I frequently want to keep the line at the current slope it's at (not equivalent the angle shown in the geometry properties, but the angle/slope on the canvas). If I grab an endpoint, I can hold shift to get 45 degree increments, but there doesn't seem to be any kind of "aspect ratio preserving" meta key I can hold to get this effect.

Basically, if I extend the line br dragging an endpoint, I'd like to be able to say, extend while preserving slope. Does this exist and I'm just missing it?