Comment 4 for bug 228971

This is already possible tanks to snapping: you can place a guide attaching it to the first point (so the origin is placed there), pressing shift while grabbing a point outside its origin you start rotating it then after releasing shift you can snap it to the second point.

The option of specifying a slope would be solved if mathematical expressions input is added to numerical fields, so you could simply write atan(<slope>)... :)

Regards.