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.

Screen shot 2011-06-11 at 9.38.46 AM

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 2011-06-11 at 8.42.48 AM

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