You can modify the slope of a line, using the contextual menu (accessible through right-click, or space bar or Option-Click)

You can also, simply, click on the Line and drag the slope wherever you want it to be. If you hold the Shift key, you can translate the line without changing the slope.

Screen Shot 2013-11-28 at 8.42.12 PM

When you select “Edit Line Slope”, a guide appears showing the new slope. When you’re satisfied, just click. The slope of the line has now been modified.

Screen Shot 2013-11-28 at 8.42.27 PM

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