Digital Light & Color

I have several colour texture images. It would help a lot if I could compare their R,G, and B histograms on the same scale (or on the same graph, similar to what RAW dialog does). I can either open 3 separate Histogram tool windows, one for each RGB channels, or use histograms in Colour Curves and switch between the RGB channels. The problem is, I think, at least in this case, that each histogram scales the y-axis automatically so there is no 'absolute' correspondence between the three. Is there a way around it i.e. how can I quantitatively compare the distribution of values in R,G, and B channels?

For what it's worth, xnView and cPicture both show R,G & B on the one histogram - no numbers, though...

Mike.