curves with straight lines

Is there a way to get a Curve correction to connect control points with straight line segments instead of the fitted smooth curve that is default? Generally this would be a bad idea, but for color correction in the a and b channels of Lab it would enable adding a single control point at 0,0 and having a discontinuity in the correction slope of the two colors on either side.

Use the Pencil tool in the Curves dialog box. Form a straight line segment between two points by clicking on the first point and Shift-clicking on the second.