To be true I started doing it yesterday and stopped around 1 am. After a few hours of minor tweaking today...
ImageViewer has a nice linear histogram option available, I also removed most of the flicker when images were redrawn using the CMemDC wrapper class.
Let me know if you feel there's a need for a logarithmic histogram.
ps. The image used is copyright Tomasz Jakobiec :)
pps. I also added a gray scale conversion filter today - looks like sepia and others may be on their way - along with some simple undo mechanism ;)