You can modify the pivot point of a line, without affecting the slope.

You can edit the pivot point by clicking on the Line while holding the shift key. Grab it and move it.

Screen Shot 2013-11-28 at 8.44.17 PM

When you select “Edit Pivot Point”, a guide appears, showing the new position. When you’re satisfied, just click. The line has been moved.

Screen Shot 2013-11-28 at 8.44.34 PM

At any time, right-clicking (or Option-click) will cancel the current operation