Thanks everyone for the quick, interesting replies. The reason for my original post was that the Live Histogram, in default mode, was showing a wider dynamic range, and a different shape for a scene than the histogram for that image in playback mode. It made me question whether I could trust what I was seeing. I followed ewavr's advise, changing the Histogram parameters to Histogram layout = Y, Ignore boundary peaks = 1. Now the Live Histogram is similar to the image histogram in playback mode. After viewing the images in Photoshop Elements, and comparing the images on screen and in camera with different histogram modes (RGB, Luminance), I'm convinced that my camera's playback histogram is in luminance mode.
snapshot