How can I show histogram of an image?

[magick]

You can use MagickLib::GetNumberColors() to write the image histogram to a file or you can iterate over the image pixels and create the histogram yourself.

[anthony]

What would be nice is to be able to generate a color map file.
That is a image that contains one color pixel for each color that exists
in the image.

EG: convert logo: -colors 32 -colormap t.gif

will return an image that is 1x32 pixels in size at the most.

This minimal image can then be used for a -map argument.
and its size can be used as a final color count.