Steve Wright, in his book, "Digital Compositing for Film and Video," uses what he calls a slice graph rather extensively to illustrate important points wrt chroma and other keying. I have seen numerous posts on the Cow as well as elsewhere with people looking for such a tool as a plugin for AE, to no avail. I would like to suggest that people check out Blender, the free, open source 3D modeling program (blender.org). The latest stable version and likely some of the previous versions contains numerous work screens one of which is the UV/Image Editor. When an image (such as a frame grab from AE) is loaded, Blender offers a 3 color histogram, waveform monitor, vectorscope, and a sample line tool. The latter is, in fact, a slice graph which can be placed and orientated arbitrarily as well as made any length. The other tools are, of course, available in Color Finesse. It takes a little work to figure out how to use Blender even for this simple task. If there is interest, I can make up a cookbook methodology for you. IF there are coders among you, the Blender code is available on-line and perhaps someone could make a plugin.
For those who'd like to go further, Blender offers many other tools of direct and specific interest to VFX and editing including a 3-D camera tracker which are not available in AE (CS-6 does have a tracker).