The Histogram option in the Image menu is purely there for informational purposes. It displays in graphical form the tonal values of the image, according to which of the channels you select. You can choose to see a histogram for either the Red, Green or Blue channel, or for all of them together by selecting Luminosity.
The graph represents the darkest to lightest pixel values (reading horizontally from left to right) between 0 and 255. There are 256 lines representing 256 possible tones. The height of the line indicates the number of pixels in the image that have that particular tone. If a tone is not present in the image it will have no associated line in the histogram. For instance, if there is no pure black in the image there will be no line at the left-hand side of the histogram, and if there is only a small number of black pixels the line will be short. The same is true of the rest of the tonal range.
This information can be useful in guiding you in your color correction strategy.
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